Supervised Machine learning:

11. What is Survivorship Bias?

It is the logical error of focusing aspects that support surviving some process and casually overlooking those that did not work because of their lack of prominence. This can lead to wrong conclusions in numerous different means.

12. What is selection Bias?

Selection bias occurs when the sample obtained is not representative of the population intended to be analysed.

13. Explain how a ROC curve works ?

The ROC curve is a graphical representation of the contrast between true positive rates and false positive rates at various thresholds. It is often used as a proxy for the trade-off between the sensitivity(true positive rate) and false positive rate.

14: The probability that an item at location A is 0.6, and 0.8 at location B. What is the probability that item would be found on Amazon website?

We need to make some assumptions about this question before we can answer it. Let’s assume that there are two possible places to purchase a particular item on Amazon and the probability of finding it at location A is 0.6 and B is 0.8. The probability of finding the item on Amazon can be explained as so:

We can reword the above as P(A) = 0.6 and P(B) = 0.8. Furthermore, let’s assume that these are independent events, meaning that the probability of one event is not impacted by the other. We can then use the formula…

P(A or B) = P(A) + P(B) — P(A and B)

P(A or B) = 0.6 + 0.8 — (0.6*0.8)

P(A or B) = 0.92

15: You randomly draw a coin from 100 coins — 1 unfair coin (head-head), 99 fair coins (head-tail) and roll it 10 times. If the result is 10 heads, what is the probability that the coin is unfair?

This can be answered using the Bayes Theorem. The extended equation for the Bayes Theorem is the following:

Assume that the probability of picking the unfair coin is denoted as P(A) and the probability of flipping 10 heads in a row is denoted as P(B). Then P(B|A) is equal to 1, P(B∣¬A) is equal to 0.⁵¹⁰, and P(¬A) is equal to 0.99.

If you fill in the equation, then P(A|B) = 0.9118 or 91.18%.

16. What is the difference between Point Estimates and Confidence Interval?

Point Estimation gives us a particular value as an estimate of a population parameter. Method of Moments and Maximum Likelihood estimator methods are used to derive Point Estimators for population parameters.

A confidence interval gives us a range of values which is likely to contain the population parameter. The confidence interval is generally preferred, as it tells us how likely this interval is to contain the population parameter. This likeliness or probability is called Confidence Level or Confidence coefficient and represented by 1 — alpha, where alpha is the level of significance.

17. What is the goal of A/B Testing?

It is a hypothesis testing for a randomized experiment with two variables A and B.

The goal of A/B Testing is to identify any changes to the web page to maximize or increase the outcome of interest. A/B testing is a fantastic method for figuring out the best online promotional and marketing strategies for your business. It can be used to test everything from website copy to sales emails to search ads

An example of this could be identifying the click-through rate for a banner ad.

18. What is p-value?

When you perform a hypothesis test in statistics, a p-value can help you determine the strength of your results. p-value is a number between 0 and 1. Based on the value it will denote the strength of the results. The claim which is on trial is called the Null Hypothesis.

Low p-value (≤ 0.05) indicates strength against the null hypothesis which means we can reject the null Hypothesis. High p-value (≥ 0.05) indicates strength for the null hypothesis which means we can accept the null Hypothesis p-value of 0.05 indicates the Hypothesis could go either way. To put it in another way,

High P values: your data are likely with a true null. Low P values: your data are unlikely with a true null.

19: Difference between convex and non-convex cost function; what does it mean when a cost function is non-convex?

A convex function is one where a line drawn between any two points on the graph lies on or above the graph. It has one minimum.

A non-convex function is one where a line drawn between any two points on the graph may intersect other points on the graph. It characterized as “wavy”.

When a cost function is non-convex, it means that there’s a likelihood that the function may find local minima instead of the global minimum, which is typically undesired in machine learning models from an optimization perspective.

20: Walk through the probability fundamentals

For this, I’m going to look at the eight rules of probability laid out here and the four different counting methods (see more here).

Eight rules of probability

• Rule #1: For any event A, 0 ≤ P(A) ≤ 1; in other words, the probability of an event can range from 0 to 1.
• Rule #2: The sum of the probabilities of all possible outcomes always equals 1.
• Rule #3: P(not A) = 1 — P(A); This rule explains the relationship between the probability of an event and its complement event. A complement event is one that includes all possible outcomes that aren’t in A.
• Rule #4: If A and B are disjoint events (mutually exclusive), then P(A or B) = P(A) + P(B); this is called the addition rule for disjoint events
• Rule #5: P(A or B) = P(A) + P(B) — P(A and B); this is called the general addition rule.
• Rule #6: If A and B are two independent events, then P(A and B) = P(A) * P(B); this is called the multiplication rule for independent events.
• Rule #7: The conditional probability of event B given event A is P(B|A) = P(A and B) / P(A)
• Rule #8: For any two events A and B, P(A and B) = P(A) * P(B|A); this is called the general multiplication rule

Counting Methods

Factorial Formula: n! = n x (n -1) x (n — 2) x … x 2 x 1

Use when the number of items is equal to the number of places available.

Eg. Find the total number of ways 5 people can sit in 5 empty seats.

= 5 x 4 x 3 x 2 x 1 = 120

Fundamental Counting Principle (multiplication)

This method should be used when repetitions are allowed and the number of ways to fill an open place is not affected by previous fills.

Eg. There are 3 types of breakfasts, 4 types of lunches, and 5 types of desserts. The total number of combinations is = 5 x 4 x 3 = 60

Permutations: P(n,r)= n! / (n−r)!

This method is used when replacements are not allowed and order of item ranking matters.

Eg. A code has 4 digits in a particular order and the digits range from 0 to 9. How many permutations are there if one digit can only be used once?

P(n,r) = 10!/(10–4)! = (10x9x8x7x6x5x4x3x2x1)/(6x5x4x3x2x1) = 5040

Combinations Formula: C(n,r)=(n!)/[(n−r)!r!]

This is used when replacements are not allowed and the order in which items are ranked does not mater.

Eg. To win the lottery, you must select the 5 correct numbers in any order from 1 to 52. What is the number of possible combinations?

C(n,r) = 52! / (52–5)!5! = 2,598,960

21: Describe Markov chains?

Brilliant provides a great definition of Markov chains (here):

“A Markov chain is a mathematical system that experiences transitions from one state to another according to certain probabilistic rules. The defining characteristic of a Markov chain is that no matter how the process arrived at its present state, the possible future states are fixed. In other words, the probability of transitioning to any particular state is dependent solely on the current state and time elapsed.”

The actual math behind Markov chains requires knowledge on linear algebra and matrices, so I’ll leave some links below in case you want to explore this topic further on your own.

22. What do you understand by statistical power of sensitivity and how do you calculate it?

Sensitivity is commonly used to validate the accuracy of a classifier (Logistic, SVM, Random Forest etc.).

Sensitivity is nothing but “Predicted True events/ Total events”. True events here are the events which were true and model also predicted them as true.

Calculation of seasonality is pretty straightforward.

Seasonality = ( True Positives ) / ( Positives in Actual Dependent Variable )

23. Why Is Re-sampling Done?

Resampling is done in any of these cases:

• Estimating the accuracy of sample statistics by using subsets of accessible data or drawing randomly with replacement from a set of data points
• Substituting labels on data points when performing significance tests
• Validating models by using random subsets (bootstrapping, cross-validation)

24. What are the differences between overfitting and under-fitting?

In statistics and machine learning, one of the most common tasks is to fit a model to a set of training data, so as to be able to make reliable predictions on general untrained data.

In overfitting, a statistical model describes random error or noise instead of the underlying relationship. Overfitting occurs when a model is excessively complex, such as having too many parameters relative to the number of observations. A model that has been overfitted, has poor predictive performance, as it overreacts to minor fluctuations in the training data.

Underfitting occurs when a statistical model or machine learning algorithm cannot capture the underlying trend of the data. Underfitting would occur, for example, when fitting a linear model to non-linear data. Such a model too would have poor predictive performance.

25. How to combat Overfitting and Underfitting?

To combat overfitting and underfitting, you can resample the data to estimate the model accuracy (k-fold cross-validation) and by having a validation dataset to evaluate the model.

26. What is regularisation? Why is it useful?

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Regularisation is the process of adding tuning parameter to a model to induce smoothness in order to prevent overfitting. This is most often done by adding a constant multiple to an existing weight vector. This constant is often the L1(Lasso) or L2(ridge). The model predictions should then minimize the loss function calculated on the regularized training set.

27. What Is the Law of Large Numbers?

It is a theorem that describes the result of performing the same experiment a large number of times. This theorem forms the basis of frequency-style thinking. It says that the sample means, the sample variance and the sample standard deviation converge to what they are trying to estimate.

28: Is mean imputation of missing data acceptable practice? Why or why not?

Mean imputation is the practice of replacing null values in a data set with the mean of the data.

Mean imputation is generally bad practice because it doesn’t take into account feature correlation. For example, imagine we have a table showing age and fitness score and imagine that an eighty-year-old has a missing fitness score. If we took the average fitness score from an age range of 15 to 80, then the eighty-year-old will appear to have a much higher fitness score that he actually should.

Second, mean imputation reduces the variance of the data and increases bias in our data. This leads to a less accurate model and a narrower confidence interval due to a smaller variance.

Machine Learning Fundamentals

29: What are some of the steps for data wrangling and data cleaning before applying machine learning algorithms?

There are many steps that can be taken when data wrangling and data cleaning. Some of the most common steps are listed below:

• Data profiling: Almost everyone starts off by getting an understanding of their dataset. More specifically, you can look at the shape of the dataset with .shape and a description of your numerical variables with .describe().
• Data visualizations: Sometimes, it’s useful to visualize your data with histograms, boxplots, and scatterplots to better understand the relationships between variables and also to identify potential outliers.
• Syntax error: This includes making sure there’s no white space, making sure letter casing is consistent, and checking for typos. You can check for typos by using .unique() or by using bar graphs.
• Standardization or normalization: Depending on the dataset your working with and the machine learning method you decide to use, it may be useful to standardize or normalize your data so that different scales of different variables don’t negatively impact the performance of your model.
• Handling null values: There are a number of ways to handle null values including deleting rows with null values altogether, replacing null values with the mean/median/mode, replacing null values with a new category (eg. unknown), predicting the values, or using machine learning models that can deal with null values. Read more here.
• Other things include: removing irrelevant data, removing duplicates, and type conversion.

30: How to deal with unbalanced binary classification?

There are a number of ways to handle unbalanced binary classification (assuming that you want to identify the minority class):

• First, you want to reconsider the metrics that you’d use to evaluate your model. The accuracy of your model might not be the best metric to look at because and I’ll use an example to explain why. Let’s say 99 bank withdrawals were not fraudulent and 1 withdrawal was. If your model simply classified every instance as “not fraudulent”, it would have an accuracy of 99%! Therefore, you may want to consider using metrics like precision and recall.
• Another method to improve unbalanced binary classification is by increasing the cost of misclassifying the minority class. By increasing the penalty of such, the model should classify the minority class more accurately.
• Lastly, you can improve the balance of classes by oversampling the minority class or by undersampling the majority class. You can read more about it here.

31: What is the difference between a box plot and a histogram?

Boxplot vs Histogram

While boxplots and histograms are visualizations used to show the distribution of the data, they communicate information differently.

Histograms are bar charts that show the frequency of a numerical variable’s values and are used to approximate the probability distribution of the given variable. It allows you to quickly understand the shape of the distribution, the variation, and potential outliers.

Boxplots communicate different aspects of the distribution of data. While you can’t see the shape of the distribution through a box plot, you can gather other information like the quartiles, the range, and outliers. Boxplots are especially useful when you want to compare multiple charts at the same time because they take up less space than histograms.

32: Describe different regularization methods, such as L1 and L2 regularization?

Both L1 and L2 regularization are methods used to reduce the overfitting of training data. Least Squares minimizes the sum of the squared residuals, which can result in low bias but high variance.

L2 Regularization, also called ridge regression, minimizes the sum of the squared residuals plus lambda times the slope squared. This additional term is called the Ridge Regression Penalty. This increases the bias of the model, making the fit worse on the training data, but also decreases the variance.

If you take the ridge regression penalty and replace it with the absolute value of the slope, then you get Lasso regression or L1 regularization.

L2 is less robust but has a stable solution and always one solution. L1 is more robust but has an unstable solution and can possibly have multiple solutions.

StatQuest has an amazing video on Lasso and Ridge regression here.

33: Neural Network Fundamentals

A neural network is a multi-layered model inspired by the human brain. Like the neurons in our brain, the circles above represent a node. The blue circles represent the input layer, the black circles represent the hidden layers, and the green circles represent the output layer. Each node in the hidden layers represents a function that the inputs go through, ultimately leading to an output in the green circles. The formal term for these functions is called the sigmoid activation function.

If you want a step by step example of creating a neural network, check out Victor Zhou’s article here.

If you’re a visual/audio learner, 3Blue1Brown has an amazing series on neural networks and deep learning on YouTube here.

34: What is cross-validation?

Cross-validation is essentially a technique used to assess how well a model performs on a new independent dataset. The simplest example of cross-validation is when you split your data into two groups: training data and testing data, where you use the training data to build the model and the testing data to test the model.

35: How to define/select metrics?

There isn’t a one-size-fits-all metric. The metric(s) chosen to evaluate a machine learning model depends on various factors:

• Is it a regression or classification task?
• What is the business objective? Eg. precision vs recall
• What is the distribution of the target variable?

There are a number of metrics that can be used, including adjusted r-squared, MAE, MSE, accuracy, recall, precision, f1 score, and the list goes on.

Check out questions related to modeling metrics on Interview Query

36: Explain what precision and recall are

Recall attempts to answer “What proportion of actual positives was identified correctly?”

Precision attempts to answer “What proportion of positive identifications was actually correct?”

Taken from Wikipedia

37: Explain what a false positive and a false negative are. Why is it important these from each other? Provide examples when false positives are more important than false negatives, false negatives are more important than false positives and when these two types of errors are equally important

A false positive is an incorrect identification of the presence of a condition when it’s absent.

A false negative is an incorrect identification of the absence of a condition when it’s actually present.

An example of when false negatives are more important than false positives is when screening for cancer. It’s much worse to say that someone doesn’t have cancer when they do, instead of saying that someone does and later realizing that they don’t.

This is a subjective argument, but false positives can be worse than false negatives from a psychological point of view. For example, a false positive for winning the lottery could be a worse outcome than a false negative because people normally don’t expect to win the lottery anyways.

38: What does NLP stand for?

NLP stands for Natural Language Processing. It is a branch of artificial intelligence that gives machines the ability to read and understand human languages.

39: When would you use random forests Vs SVM and why?

There are a couple of reasons why a random forest is a better choice of model than a support vector machine:

• Random forests allow you to determine the feature importance. SVM’s can’t do this.
• Random forests are much quicker and simpler to build than an SVM.
• For multi-class classification problems, SVMs require a one-vs-rest method, which is less scalable and more memory intensive.

40: Why is dimension reduction important?

Dimensionality reduction is the process of reducing the number of features in a dataset. This is important mainly in the case when you want to reduce variance in your model (overfitting).

Wikipedia states four advantages of dimensionality reduction (see here):

1. It reduces the time and storage space required
2. Removal of multi-collinearity improves the interpretation of the parameters of the machine learning model
3. It becomes easier to visualize the data when reduced to very low dimensions such as 2D or 3D
4. It avoids the curse of dimensionality

41: What is principal component analysis? Explain the sort of problems you would use PCA for.

In its simplest sense, PCA involves project higher dimensional data (eg. 3 dimensions) to a smaller space (eg. 2 dimensions). This results in a lower dimension of data, (2 dimensions instead of 3 dimensions) while keeping all original variables in the model.

PCA is commonly used for compression purposes, to reduce required memory and to speed up the algorithm, as well as for visualization purposes, making it easier to summarize data.

42: Why is Naive Bayes so bad? How would you improve a spam detection algorithm that uses naive Bayes?

One major drawback of Naive Bayes is that it holds a strong assumption in that the features are assumed to be uncorrelated with one another, which typically is never the case.

One way to improve such an algorithm that uses Naive Bayes is by decorrelating the features so that the assumption holds true.

43: What are the drawbacks of a linear model?

There are a couple of drawbacks of a linear model:

• A linear model holds some strong assumptions that may not be true in application. It assumes a linear relationship, multivariate normality, no or little multicollinearity, no auto-correlation, and homoscedasticity
• A linear model can’t be used for discrete or binary outcomes.
• You can’t vary the model flexibility of a linear model.

44: Do you think 50 small decision trees are better than a large one? Why?

Another way of asking this question is “Is a random forest a better model than a decision tree?” And the answer is yes because a random forest is an ensemble method that takes many weak decision trees to make a strong learner. Random forests are more accurate, more robust, and less prone to overfitting.

45: Why is mean square error a bad measure of model performance? What would you suggest instead?

Mean Squared Error (MSE) gives a relatively high weight to large errors — therefore, MSE tends to put too much emphasis on large deviations. A more robust alternative is MAE (mean absolute deviation).

46: What are the assumptions required for linear regression? What if some of these assumptions are violated?

The assumptions are as follows:

1. The sample data used to fit the model is representative of the population
2. The relationship between X and the mean of Y is linear
3. The variance of the residual is the same for any value of X (homoscedasticity)
4. Observations are independent of each other
5. For any value of X, Y is normally distributed.

Extreme violations of these assumptions will make the results redundant. Small violations of these assumptions will result in a greater bias or variance of the estimate.

47: What is collinearity and what to do with it? How to remove multicollinearity?

Multicollinearity exists when an independent variable is highly correlated with another independent variable in a multiple regression equation. This can be problematic because it undermines the statistical significance of an independent variable.

You could use the Variance Inflation Factors (VIF) to determine if there is any multicollinearity between independent variables — a standard benchmark is that if the VIF is greater than 5 then multicollinearity exists.

48: How to check if the regression model fits the data well?

There are a couple of metrics that you can use:

R-squared/Adjusted R-squared: Relative measure of fit. This was explained in a previous answer

F1 Score: Evaluates the null hypothesis that all regression coefficients are equal to zero vs the alternative hypothesis that at least one doesn’t equal zero

RMSE: Absolute measure of fit.

49: What is a decision tree?

Decision trees are a popular model, used in operations research, strategic planning, and machine learning. Each square above is called a node, and the more nodes you have, the more accurate your decision tree will be (generally). The last nodes of the decision tree, where a decision is made, are called the leaves of the tree. Decision trees are intuitive and easy to build but fall short when it comes to accuracy.

50: What is a random forest? Why is it good?

Random forests are an ensemble learning technique that builds off of decision trees. Random forests involve creating multiple decision trees using bootstrapped datasets of the original data and randomly selecting a subset of variables at each step of the decision tree. The model then selects the mode of all of the predictions of each decision tree. By relying on a “majority wins” model, it reduces the risk of error from an individual tree.

For example, if we created one decision tree, the third one, it would predict 0. But if we relied on the mode of all 4 decision trees, the predicted value would be 1. This is the power of random forests.

Random forests offer several other benefits including strong performance, can model non-linear boundaries, no cross-validation needed, and gives feature importance.

51: What is a kernel? Explain the kernel trick

A kernel is a way of computing the dot product of two vectors 𝐱x and 𝐲y in some (possibly very high dimensional) feature space, which is why kernel functions are sometimes called “generalized dot product” [2]

The kernel trick is a method of using a linear classifier to solve a non-linear problem by transforming linearly inseparable data to linearly separable ones in a higher dimension.

52: Is it beneficial to perform dimensionality reduction before fitting an SVM? Why or why not?

When the number of features is greater than the number of observations, then performing dimensionality reduction will generally improve the SVM.

53: What is overfitting?

Overfitting is an error where the model ‘fits’ the data too well, resulting in a model with high variance and low bias. As a consequence, an overfit model will inaccurately predict new data points even though it has a high accuracy on the training data.

54: What is boosting?

Boosting is an ensemble method to improve a model by reducing its bias and variance, ultimately converting weak learners to strong learners. The general idea is to train a weak learner and sequentially iterate and improve the model by learning from the previous learner. You can learn more about it here.

DEEP LEARNING INTERVIEW QUESTIONS

55. What do you mean by Deep Learning? 

Deep Learning is nothing but a paradigm of machine learning which has shown incredible promise in recent years. This is because of the fact that Deep Learning shows a great analogy with the functioning of the human brain.

56. What is the difference between machine learning and deep learning?

Machine learning is a field of computer science that gives computers the ability to learn without being explicitly programmed. Machine learning can be categorised in the following three categories.

1. Supervised machine learning,
2. Unsupervised machine learning,
3. Reinforcement learning

Deep Learning is a subfield of machine learning concerned with algorithms inspired by the structure and function of the brain called artificial neural networks.

57. What, in your opinion, is the reason for the popularity of Deep Learning in recent times?

Now although Deep Learning has been around for many years, the major breakthroughs from these techniques came just in recent years. This is because of two main reasons:

• The increase in the amount of data generated through various sources
• The growth in hardware resources required to run these models

GPUs are multiple times faster and they help us build bigger and deeper deep learning models in comparatively less time than we required previously.

58.what is reinforcement learning?

Reinforcement Learning is learning what to do and how to map situations to actions. The end result is to maximise the numerical reward signal. The learner is not told which action to take but instead must discover which action will yield the maximum reward. Reinforcement learning is inspired by the learning of human beings, it is based on the reward/penalty mechanism.

59. What are Artificial Neural Networks?

Artificial Neural networks are a specific set of algorithms that have revolutionized machine learning. They are inspired by biological neural networks. Neural Networks can adapt to changing the input so the network generates the best possible result without needing to redesign the output criteria.

60. Describe the structure of Artificial Neural Networks?

Artificial Neural Networks works on the same principle as a biological Neural Network. It consists of inputs which get processed with weighted sums and Bias, with the help of Activation Functions.

61. How Are Weights Initialized in a Network?

There are two methods here: we can either initialize the weights to zero or assign them randomly.

Initializing all weights to 0: This makes your model similar to a linear model. All the neurons and every layer perform the same operation, giving the same output and making the deep net useless.

Initializing all weights randomly: Here, the weights are assigned randomly by initializing them very close to 0. It gives better accuracy to the model since every neuron performs different computations. This is the most commonly used method.

62. What Is the Cost Function?

Also referred to as “loss” or “error,” cost function is a measure to evaluate how good your model’s performance is. It’s used to compute the error of the output layer during backpropagation. We push that error backwards through the neural network and use that during the different training functions.

63. What Are Hyperparameters?

With neural networks, you’re usually working with hyperparameters once the data is formatted correctly. A hyperparameter is a parameter whose value is set before the learning process begins. It determines how a network is trained and the structure of the network (such as the number of hidden units, the learning rate, epochs, etc.).

64. What Will Happen If the Learning Rate Is Set inaccurately (Too Low or Too High)?

When your learning rate is too low, training of the model will progress very slowly as we are making minimal updates to the weights. It will take many updates before reaching the minimum point.

If the learning rate is set too high, this causes undesirable divergent behaviour to the loss function due to drastic updates in weights. It may fail to converge (model can give a good output) or even diverge (data is too chaotic for the network to train).

65. What Is the Difference Between Epoch, Batch, and Iteration in Deep Learning?

• Epoch – Represents one iteration over the entire dataset (everything put into the training model).
• Batch – Refers to when we cannot pass the entire dataset into the neural network at once, so we divide the dataset into several batches.
• Iteration – if we have 10,000 images as data and a batch size of 200. then an epoch should run 50 iterations (10,000 divided by 50).

66. What Are the Different Layers on CNN?

There are four layers in CNN:

1. Convolutional Layer –  the layer that performs a convolutional operation, creating several smaller picture windows to go over the data.
2. ReLU Layer – it brings non-linearity to the network and converts all the negative pixels to zero. The output is a rectified feature map.
3. Pooling Layer – pooling is a down-sampling operation that reduces the dimensionality of the feature map.
4. Fully Connected Layer – this layer recognizes and classifies the objects in the image.

67. What Is Pooling on CNN, and How Does It Work?

Pooling is used to reduce the spatial dimensions of a CNN. It performs down-sampling operations to reduce the dimensionality and creates a pooled feature map by sliding a filter matrix over the input matrix.

68. What are Recurrent Neural Networks(RNNs)?

RNNs are a type of artificial neural networks designed to recognise the pattern from the sequence of data such as Time series, stock market and government agencies etc. To understand recurrent nets, first, you have to understand the basics of feedforward nets.

Both these networks RNN and feed-forward named after the way they channel information through a series of mathematical orations performed at the nodes of the network. One feeds information through straight(never touching the same node twice), while the other cycles it through a loop, and the latter are called recurrent.

Recurrent networks, on the other hand, take as their input, not just the current input example they see, but also the what they have perceived previously in time.

The decision a recurrent neural network reached at time t-1 affects the decision that it will reach one moment later at time t. So recurrent networks have two sources of input, the present and the recent past, which combine to determine how they respond to new data, much as we do in life.

The error they generate will return via backpropagation and be used to adjust their weights until error can’t go any lower. Remember, the purpose of recurrent nets is to accurately classify sequential input. We rely on the backpropagation of error and gradient descent to do so.

69. How Does an LSTM Network Work?

Long-Short-Term Memory (LSTM) is a special kind of recurrent neural network capable of learning long-term dependencies, remembering information for long periods as its default behaviour. There are three steps in an LSTM network:

• Step 1: The network decides what to forget and what to remember.
• Step 2: It selectively updates cell state values.
• Step 3: The network decides what part of the current state makes it to the output.

70. What Is a Multi-layer Perceptron(MLP)?

As in Neural Networks, MLPs have an input layer, a hidden layer, and an output layer. It has the same structure as a single layer perceptron with one or more hidden layers. A single layer perceptron can classify only linear separable classes with binary output (0,1), but MLP can classify nonlinear classes.

Except for the input layer, each node in the other layers uses a nonlinear activation function. This means the input layers, the data coming in, and the activation function is based upon all nodes and weights being added together, producing the output. MLP uses a supervised learning method called “backpropagation.” In backpropagation, the neural network calculates the error with the help of cost function. It propagates this error backward from where it came (adjusts the weights to train the model more accurately).

71. Explain Gradient Descent.

To Understand Gradient Descent, Let’s understand what is a Gradient first.

A gradient measures how much the output of a function changes if you change the inputs a little bit. It simply measures the change in all weights with regard to the change in error. You can also think of a gradient as the slope of a function.

Gradient Descent can be thought of climbing down to the bottom of a valley, instead of climbing up a hill.  This is because it is a minimization algorithm that minimizes a given function (Activation Function).

72. What are vanishing gradients?

While training an RNN, your slope can become either too small; this makes the training difficult. When the slope is too small, the problem is known as a Vanishing Gradient. It leads to long training times, poor performance, and low accuracy.

73. What is Back Propagation and Explain it’s Working.

Backpropagation is a training algorithm used for multilayer neural network. In this method, we move the error from an end of the network to all weights inside the network and thus allowing efficient computation of the gradient.

It has the following steps:

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• Forward Propagation of Training Data
• Derivatives are computed using output and target
• Back Propagate for computing derivative of error wrt output activation
• Using previously calculated derivatives for output
• Update the Weights

74. What are the variants of Back Propagation?

• Stochastic Gradient Descent: We use only a single training example for calculation of gradient and update parameters.
• Batch Gradient Descent: We calculate the gradient for the whole dataset and perform the update at each iteration.
• Mini-batch Gradient Descent: It’s one of the most popular optimization algorithms. It’s a variant of Stochastic Gradient Descent and here instead of single training example, mini-batch of samples is used.

75. What are the different Deep Learning Frameworks?

• Pytorch
• TensorFlow
• Microsoft Cognitive Toolkit
• Keras
• Caffe
• Chainer

76. What is the role of the Activation Function?

The Activation function is used to introduce non-linearity into the neural network helping it to learn more complex function. Without which the neural network would be only able to learn linear function which is a linear combination of its input data. An activation function is a function in an artificial neuron that delivers an output based on inputs.

77. Name a few Machine Learning libraries for various purposes.

Purpose	Libraries
Scientific Computation	Numpy
Tabular Data	Pandas
Data Modelling & Preprocessing	Scikit Learn
Time-Series Analysis	Statsmodels
Text processing	Regular Expressions, NLTK
Deep Learning	Tensorflow, Pytorch

78. What is an Auto-Encoder? 

Auto-encoders are simple learning networks that aim to transform inputs into outputs with the minimum possible error. This means that we want the output to be as close to input as possible. We add a couple of layers between the input and the output, and the sizes of these layers are smaller than the input layer. The auto-encoder receives unlabelled input which is then encoded to reconstruct the input.

79. What is a Boltzmann Machine? 

Boltzmann machines have a simple learning algorithm that allows them to discover interesting features that represent complex regularities in the training data. The Boltzmann machine is basically used to optimise the weights and the quantity for the given problem. The learning algorithm is very slow in networks with many layers of feature detectors. “Restricted Boltzmann Machines” algorithm has a single layer of feature detectors which makes it faster than the rest.

80. What Is Dropout and Batch Normalization?

Dropout is a technique of dropping out hidden and visible units of a network randomly to prevent overfitting of data (typically dropping 20 per cent of the nodes). It doubles the number of iterations needed to converge the network.

Batch normalization is the technique to improve the performance and stability of neural networks by normalizing the inputs in every layer so that they have mean output activation of zero and standard deviation of one.

81. What Is the Difference Between Batch Gradient Descent and Stochastic Gradient Descent?

Batch Gradient Descent	Stochastic Gradient Descent
The batch gradient computes the gradient using the entire dataset.	The stochastic gradient computes the gradient using a single sample.
It takes time to converge because the volume of data is huge, and weights update slowly.	It converges much faster than the batch gradient because it updates weight more frequently.

82. Why Is Tensorflow the Most Preferred Library in Deep Learning?

Tensorflow provides both C++ and Python APIs, making it easier to work on and has a faster compilation time compared to other Deep Learning libraries like Keras and Torch. Tensorflow supports both CPU and GPU computing devices.

83. What Do You Mean by Tensor in Tensorflow?

A tensor is a mathematical object represented as arrays of higher dimensions. These arrays of data with different dimensions and ranks fed as input to the neural network are called “Tensors.”

84. What is the Computational Graph?

Everything in a tensorflow is based on creating a computational graph. It has a network of nodes where each node operates, Nodes represent mathematical operations, and edges represent tensors. Since data flows in the form of a graph, it is also called a “DataFlow Graph.”

85. What is a Generative Adversarial Network?

Suppose there is a wine shop purchasing wine from dealers, which they resell later. But some dealers sell fake wine. In this case, the shop owner should be able to distinguish between fake and authentic wine.

The forger will try different techniques to sell fake wine and make sure specific techniques go past the shop owner’s check. The shop owner would probably get some feedback from wine experts that some of the wine is not original. The owner would have to improve how he determines whether a wine is fake or authentic.

The forger’s goal is to create wines that are indistinguishable from the authentic ones while the shop owner intends to tell if the wine is real or not accurately

Let us understand this example with the help of an image.

There is a noise vector coming into the forger who is generating fake wine.

Here the forger acts as a Generator.

The shop owner acts as a Discriminator.

The Discriminator gets two inputs; one is the fake wine, while the other is the real authentic wine. The shop owner has to figure out whether it is real or fake.

So, there are two primary components of Generative Adversarial Network (GAN) named:

1. Generator
2. Discriminator

The generator is a CNN that keeps keys producing images and is closer in appearance to the real images while the discriminator tries to determine the difference between real and fake images The ultimate aim is to make the discriminator learn to identify real and fake images.

Apart from the very technical questions, your interviewer could even hit you up with a few simple ones to check your overall confidence, in the likes of the following.

86. What are the important skills to have in Python with regard to data analysis?

The following are some of the important skills to possess which will come handy when performing data analysis using Python.

• Good understanding of the built-in data types especially lists, dictionaries, tuples, and sets.
• Mastery of N-dimensional NumPy Arrays.
• Mastery of Pandas dataframes.
• Ability to perform element-wise vector and matrix operations on NumPy arrays.
• Knowing that you should use the Anaconda distribution and the conda package manager.
• Familiarity with Scikit-learn. **Scikit-Learn Cheat Sheet**
• Ability to write efficient list comprehensions instead of traditional for loops.
• Ability to write small, clean functions (important for any developer), preferably pure functions that don’t alter objects.
• Knowing how to profile the performance of a Python script and how to optimize bottlenecks.

The following will help to tackle any problem in data analytics and machine learning.

87. What is a Box Cox Transformation?

Dependent variable for a regression analysis might not satisfy one or more assumptions of an ordinary least squares regression. The residuals could either curve as the prediction increases or follow skewed distribution. In such scenarios, it is necessary to transform the response variable so that the data meets the required assumptions. A Box cox transformation is a statistical technique to transform non-normal dependent variables into a normal shape. If the given data is not normal then most of the statistical techniques assume normality. Applying a box cox transformation means that you can run a broader number of tests.

A Box Cox transformation is a way to transform non-normal dependent variables into a normal shape. Normality is an important assumption for many statistical techniques, if your data isn’t normal, applying a Box-Cox means that you are able to run a broader number of tests. The Box Cox transformation is named after statisticians George Box and Sir David Roxbee Cox who collaborated on a 1964 paper and developed the technique.

88: Explain what a false positive and a false negative are. Why is it important for each other? Provide examples when false positives are more important than false negatives, false negatives are more important than false positives and when these two types of errors are equally important

A false positive is an incorrect identification of the presence of a condition when it’s absent.

A false negative is an incorrect identification of the absence of a condition when it’s actually present.

An example of when false negatives are more important than false positives is when screening for cancer. It’s much worse to say that someone doesn’t have cancer when they do, instead of saying that someone does and later realizing that they don’t.

This is a subjective argument, but false positives can be worse than false negatives from a psychological point of view. For example, a false positive for winning the lottery could be a worse outcome than a false negative because people normally don’t expect to win the lottery anyways.

89: Do you think 50 small decision trees are better than a large one? Why?

Another way of asking this question is “Is a random forest a better model than a decision tree?” And the answer is yes because a random forest is an ensemble method that takes many weak decision trees to make a strong learner. Random forests are more accurate, more robust, and less prone to overfitting.

90: What is collinearity and what to do with it? How to remove multicollinearity?

Multicollinearity exists when an independent variable is highly correlated with another independent variable in a multiple regression equation. This can be problematic because it undermines the statistical significance of an independent variable.

You could use the Variance Inflation Factors (VIF) to determine if there is any multicollinearity between independent variables — a standard benchmark is that if the VIF is greater than 5 then multicollinearity exists.

91: How to check if the regression model fits the data well?

There are a couple of metrics that you can use:

R-squared/Adjusted R-squared: Relative measure of fit. This was explained in a previous answer

F1 Score: Evaluates the null hypothesis that all regression coefficients are equal to zero vs the alternative hypothesis that at least one doesn’t equal zero

RMSE: Absolute measure of fit.

92. During analysis, how do you treat missing values?

The extent of the missing values is identified after identifying the variables with missing values. If any patterns are identified the analyst has to concentrate on them as it could lead to interesting and meaningful business insights.

If there are no patterns identified, then the missing values can be substituted with mean or median values (imputation) or they can simply be ignored. Assigning a default value which can be mean, minimum or maximum value. Getting into the data is important.

If it is a categorical variable, the default value is assigned. The missing value is assigned a default value. If you have a distribution of data coming, for normal distribution give the mean value.

If 80% of the values for a variable are missing then you can answer that you would be dropping the variable instead of treating the missing values.

93: Give examples of data that does not have a Gaussian distribution, nor log-normal.

• Any type of categorical data won’t have a gaussian distribution or lognormal distribution.
• Exponential distributions — eg. the amount of time that a car battery lasts or the amount of time until an earthquake occurs.

94: What is root cause analysis? How to identify a cause vs. a correlation? Give examples

Root cause analysis: a method of problem-solving used for identifying the root cause(s) of a problem [5]

Correlation measures the relationship between two variables, range from -1 to 1. Causation is when a first event appears to have caused a second event. Causation essentially looks at direct relationships while correlation can look at both direct and indirect relationships.

Example: a higher crime rate is associated with higher sales in ice cream in Canada, aka they are positively correlated. However, this doesn’t mean that one causes another. Instead, it’s because both occur more when it’s warmer outside.

You can test for causation using hypothesis testing or A/B testing.

95: Give an example where the median is a better measure than the mean

When there are a number of outliers that positively or negatively skew the data.

96: Given two fair dices, what is the probability of getting scores that sum to 4? to 8?

There are 4 combinations of rolling a 4 (1+3, 3+1, 2+2):

P(rolling a 4) = 3/36 = 1/12

There are combinations of rolling an 8 (2+6, 6+2, 3+5, 5+3, 4+4):

P(rolling an 8) = 5/36

97. How do you calculate the needed sample size?

Formula for margin of error

You can use the margin of error (ME) formula to determine the desired sample size.

• t/z = t/z score used to calculate the confidence interval
• ME = the desired margin of error
• S = sample standard deviation

98: Infection rates at a hospital above a 1 infection per 100 person-days at risk are considered high. A hospital had 10 infections over the last 1787 person-days at risk. Give the p-value of the correct one-sided test of whether the hospital is below the standard.

Since we looking at the number of events (# of infections) occurring within a given timeframe, this is a Poisson distribution question.

The probability of observing k events in an interval

Null (H0): 1 infection per person-days

Alternative (H1): >1 infection per person-days

k (actual) = 10 infections

lambda (theoretical) = (1/100)*1787

p = 0.032372 or 3.2372% calculated using .poisson() in excel or ppois in R

Since p-value < alpha (assuming 5% level of significance), we reject the null and conclude that the hospital is below the standard.

99: You roll a biased coin (p(head)=0.8) five times. What’s the probability of getting three or more heads?

Use the General Binomial Probability formula to answer this question:

General Binomial Probability Formula

p = 0.8

n = 5

k = 3,4,5

P(3 or more heads) = P(3 heads) + P(4 heads) + P(5 heads) = 0.94 or 94%

100: A random variable X is normal with mean 1020 and a standard deviation 50. Calculate P(X>1200)

Using Excel…

p =1-norm.dist(1200, 1020, 50, true)

p= 0.000159

101: Consider the number of people that show up at a bus station is Poisson with mean 2.5/h. What is the probability that at most three people show up in a four hour period?

x = 3

mean = 2.5*4 = 10

using Excel…

p = poisson.dist(3,10,true)

p = 0.010336

102: An HIV test has a sensitivity of 99.7% and a specificity of 98.5%. A subject from a population of prevalence 0.1% receives a positive test result. What is the precision of the test (i.e the probability he is HIV positive)?

Equation for Precision (PV)

Precision = Positive Predictive Value = PV

PV = (0.001*0.997)/[(0.001*0.997)+((1–0.001)*(1–0.985))]

PV = 0.0624 or 6.24%

See more about this equation here.

103: You are running for office and your pollster polled hundred people. Sixty of them claimed they will vote for you. Can you relax?

• Assume that there’s only you and one other opponent.
• Also, assume that we want a 95% confidence interval. This gives us a z-score of 1.96.
  

Confidence interval formula

p-hat = 60/100 = 0.6

z* = 1.96

n = 100

This gives us a confidence interval of [50.4,69.6]. Therefore, given a confidence interval of 95%, if you are okay with the worst scenario of tying then you can relax. Otherwise, you cannot relax until you got 61 out of 100 to claim yes.

104: Geiger counter records 100 radioactive decays in 5 minutes. Find an approximate 95% interval for the number of decays per hour.

• Since this is a Poisson distribution question, mean = lambda = variance, which also means that standard deviation = square root of the mean
• a 95% confidence interval implies a z score of 1.96
• one standard deviation = 10

Therefore the confidence interval = 100 +/- 19.6 = [964.8, 1435.2]

105: Consider influenza epidemics for two-parent heterosexual families. Suppose that the probability is 17% that at least one of the parents has contracted the disease. The probability that the father has contracted influenza is 12% while the probability that both the mother and father have contracted the disease is 6%. What is the probability that the mother has contracted influenza?

Using the General Addition Rule in probability:

P(mother or father) = P(mother) + P(father) — P(mother and father)

P(mother) = P(mother or father) + P(mother and father) — P(father)

P(mother) = 0.17 + 0.06–0.12

P(mother) = 0.11

106: Suppose that diastolic blood pressures (DBPs) for men aged 35–44 are normally distributed with a mean of 80 (mm Hg) and a standard deviation of 10. About what is the probability that a random 35–44 year old has a DBP less than 70?

Since 70 is one standard deviation below the mean, take the area of the Gaussian distribution to the left of one standard deviation.

= 2.3 + 13.6 = 15.9%

107: In a population of interest, a sample of 9 men yielded a sample average brain volume of 1,100cc and a standard deviation of 30cc. What is a 95% Student’s T confidence interval for the mean brain volume in this new population?

Confidence interval for sample

Given a confidence level of 95% and degrees of freedom equal to 8, the t-score = 2.306

Confidence interval = 1100 +/- 2.306*(30/3)

Confidence interval = [1076.94, 1123.06]

108: A diet pill is given to 9 subjects over six weeks. The average difference in weight (follow up — baseline) is -2 pounds. What would the standard deviation of the difference in weight have to be for the upper endpoint of the 95% T confidence interval to touch 0?

Upper bound = mean + t-score*(standard deviation/sqrt(sample size))

0 = -2 + 2.306*(s/3)

2 = 2.306 * s / 3

s = 2.601903

Therefore the standard deviation would have to be at least approximately 2.60 for the upper bound of the 95% T confidence interval to touch 0.

109: In a study of emergency room waiting times, investigators consider a new and the standard triage systems. To test the systems, administrators selected 20 nights and randomly assigned the new triage system to be used on 10 nights and the standard system on the remaining 10 nights. They calculated the nightly median waiting time (MWT) to see a physician. The average MWT for the new system was 3 hours with a variance of 0.60 while the average MWT for the old system was 5 hours with a variance of 0.68. Consider the 95% confidence interval estimate for the differences of the mean MWT associated with the new system. Assume a constant variance. What is the interval? Subtract in this order (New System — Old System).

See here for full tutorial on finding the Confidence Interval for Two Independent Samples.

Confidence Interval = mean +/- t-score * standard error (see above)

mean = new mean — old mean = 3–5 = -2

t-score = 2.101 given df=18 (20–2) and confidence interval of 95%

standard error = sqrt((0.⁶²*9+0.⁶⁸²*9)/(10+10–2)) * sqrt(1/10+1/10)

standard error = 0.352

confidence interval = [-2.75, -1.25]

This exhaustive list is sure to strengthen your preparation for data science interview questions.

Stay Sharp with Our Data Science Interview Questions

For data scientists, the work isn’t easy, but it’s rewarding and there are plenty of available positions out there. These data science interview questions can help you get one step closer to your dream job. So, prepare yourself for the rigors of interviewing and stay sharp with the nuts and bolts of data science.

I hope this set of Data Science Interview Questions and Answers will help you in preparing for your interviews. All the best!

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