100 Common Data Science Interview Questions & Answers
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In collaboration with data scientists, industry experts, and top counselors, we have put together a list of general data science interview questions and answers to help you with your preparation in applying for data science jobs. This first part of a series of data science interview questions and answers article focuses only on common topics like questions around data, probability, statistics, and other data science concepts. This also includes a list of openended questions that interviewers ask to get a feel of how often and how quickly you can think on your feet. There are some data analyst interview questions in this blog that can also be asked in a data science interview. These kinds of analytics interview questions are asked to measure if you were successful in applying data science techniques to reallife problems.
Data Science is not an easy field to get into. This is something all data scientists will agree on. Apart from having a degree in mathematics/statistics or engineering, a data scientist also needs to go through intense training to develop all the skills required for this field. Apart from the degree/diploma and the training, it is important to prepare the right resume for a data science job and to be well versed with the data science interview questions and answers.
Consider our top 100 Data Science Interview Questions and Answers as a starting point for your data scientist interview preparation. Even if you are not looking for a data scientist position now, as you are still working your way through handson projects and learning programming languages like Python and R – you can start practicing these Data Scientist Interview questions and answers. These Data Scientist job interview questions will set the foundation for data science interviews to impress potential employers by knowing about your subject and being able to show the practical implications of data science.
1)Differentiate between Data Science, Machine Learning, and AI.
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Data Science vs Machine Learning
Criteria 
Data Science 
Machine Learning 
Artificial Intelligence 
Definition  Data Science is not exactly a subset of machine learning but it uses machine learning to analyze and make future predictions.  A subset of AI that focuses on a narrow range of activities.  A wide term that focuses on applications ranging from Robotics to Text Analysis. 
Role  It can take on a business role.  It is a purely technical role.  It is a combination of both business and technical aspects. 
Scope  Data Science is a broad term for diverse disciplines and is not merely about developing and training models.  Machine learning fits within the data science spectrum.  AI is a subfield of computer science. 
AI  Loosely integrated  Machine learning is a subfield of AI and is tightly integrated.  A subfield of computer science consisting of various tasks like planning, moving around in the world, recognizing objects and sounds, speaking, translating, performing social or business transactions, creative work. 
2) Python or R – Which one would you prefer for text analytics?
The best possible answer for this would be Python because it has a Pandas library that provides easytouse data structures and highperformance data analysis tools.
3) Which technique is used to predict categorical responses?
Classification technique is used widely in mining for classifying data sets.
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4) What is logistic regression? Or State an example when you have used logistic regression recently. (Access a sample usecase on Logistic Regression)
Logistic Regression often referred to as the logit model is a technique to predict the binary outcome from a linear combination of predictor variables. For example, if you want to predict whether a particular political leader will win the election or not. In this case, the outcome of prediction is binary i.e. 0 or 1 (Win/Lose). The predictor variables here would be the amount of money spent for election campaigning of a particular candidate, the amount of time spent in campaigning, etc.
5) What are Recommender Systems?
A subclass of information filtering systems that are meant to predict the preferences or ratings that a user would give to a product. Recommender systems are widely used in movies, news, research articles, products, social tags, music, etc.
6) Why data cleaning plays a vital role in the analysis? (Access popular Python and R Codes for data cleaning )
Cleaning data from multiple sources to transform it into a format that data analysts or data scientists can work with is a cumbersome process because  as the number of data sources increases, the time take to clean the data increases exponentially due to the number of sources and the volume of data generated in these sources. It might take up to 80% of the time for just cleaning data making it a critical part of the analysis task.
7) Differentiate between univariate, bivariate, and multivariate analysis.
These are descriptive statistical analysis techniques that can be differentiated based on the number of variables involved at a given point in time. For example, the pie charts of sales based on territory involve only one variable and can be referred to as univariate analysis.
If the analysis attempts to understand the difference between 2 variables at the time as in a scatterplot, then it is referred to as bivariate analysis. For example, analyzing the volume of sales and spending can be considered as an example of bivariate analysis.
Analysis that deals with the study of more than two variables to understand the effect of variables on the responses is referred to as multivariate analysis.
8) What do you understand by the term Normal Distribution? (Learn how to plot normal distribution using Seaborn)
Data is usually distributed in different ways with a bias to the left or to the right or it can all be jumbled up. However, there are chances that data is distributed around a central value without any bias to the left or right and reaches normal distribution in the form of a bellshaped curve. The random variables are distributed in the form of a symmetrical bell shaped curve.
Image Credit: mathisfun.com
9) What is Linear Regression?
Linear regression is a statistical technique where the score of a variable Y is predicted from the score of a second variable X. X is referred to as the predictor variable and Y as the criterion variable.
10) What are Interpolation and Extrapolation?
Estimating a value from 2 known values from a list of values is Interpolation. Extrapolation is approximating a value by extending a known set of values or facts.
11) What is power analysis?
An experimental design technique for determining the effect of a given sample size.
12) What is Kmeans? How can you select K for Kmeans?
13) What is Collaborative filtering?
The process of filtering used by most of the recommender systems to find patterns or information by collaborating viewpoints, various data sources, and multiple agents.
14) What is the difference between Cluster and Systematic Sampling?
Cluster sampling is a technique used when it becomes difficult to study the target population spread across a wide area and simple random sampling cannot be applied. A cluster sample is a probability sample where each sampling unit is a collection or cluster of elements. Systematic sampling is a statistical technique where elements are selected from an ordered sampling frame. In systematic sampling, the list is progressed in a circular manner so once you reach the end of the list, it is progressed from the top again. The best example for systematic sampling is the equal probability method.
15) Are expected value and mean value different?
They are not different but the terms are used in different contexts. Mean is generally referred to when talking about a probability distribution or sample population whereas expected value is generally referred in a random variable context.
For Sampling Data
The mean value is the only value that comes from the sampling data.
Expected Value is the mean of all the means i.e. the value that is built from multiple samples. The expected value is the population mean.
For Distributions
Mean value and Expected value are the same irrespective of the distribution, under the condition that the distribution is in the same population.
16) What does Pvalue signify about the statistical data?
Pvalue is used to determine the significance of results after a hypothesis test in statistics. Pvalue helps the readers to draw conclusions and is always between 0 and 1.
• PValue > 0.05 denotes weak evidence against the null hypothesis which means the null hypothesis cannot be rejected.
• Pvalue <= 0.05 denotes strong evidence against the null hypothesis which means the null hypothesis can be rejected.
• Pvalue=0.05is the marginal value indicating it is possible to go either way.
17) Do gradient descent methods always converge to the same point?
No, they do not because in some cases it reaches a local minima or a local optima point. You don’t reach the global optima point. It depends on the data and starting conditions
18) What is the benefit of shuffling a training dataset when using a batch gradient descent algorithm for optimizing a neural network?
19) A test has a true positive rate of 100% and falsepositive rate of 5%. There is a population with a 1/1000 rate of having the condition the test identifies. Considering a positive test, what is the probability of having that condition?
Let’s suppose you are being tested for a disease, if you have the illness the test will end up saying you have the illness. However, if you don’t have the illness 5% of the time the test will end up saying you have the illness, and 95% of the time the test will give an accurate result that you don’t have the illness. Thus there is a 5% error in case you do not have the illness.
Out of 1000 people, 1 person who has the disease will get a true positive result.
Out of the remaining 999 people, 5% will also get true positive results.
Close to 50 people will get a true positive result for the disease.
This means that out of 1000 people, 51 people will be tested positive for the disease even though only one person has the illness. There is only a 2% probability of you having the disease even if your reports say that you have the disease.
20) How you can make data normal using BoxCox transformation?
21) What is the difference between Supervised Learning and Unsupervised Learning?
If an algorithm learns something from the training data so that the knowledge can be applied to the test data, then it is referred to as Supervised Learning. Classification is an example of Supervised Learning. If the algorithm does not learn anything beforehand because there is no response variable or any training data, then it is referred to as unsupervised learning. Clustering is an example of unsupervised learning.
22) Why it is not advisable to use a softmax output activation function in a multilabel classification problem for a onehot encoded target?
23) Why is vectorization considered a powerful method for optimizing numerical code?
24) What is the goal of A/B Testing?
It is a statistical 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. An example of this could be identifying the clickthrough rate for a banner ad.
25) What is an Eigenvalue and Eigenvector?
Eigenvectors are used for understanding linear transformations. In data analysis, we usually calculate the eigenvectors for a correlation or covariance matrix. Eigenvectors are the directions along which a particular linear transformation acts by flipping, compressing, or stretching. Eigenvalue can be referred to as the strength of the transformation in the direction of the eigenvector or the factor by which the compression occurs.
26) What is Gradient Descent?
27) Differentiate between a multilabel classification problem and a multiclass classification problem.
28) What is the difference between gradient descent optimization algorithms Adam and Momentum?
29) What are various steps involved in an analytics project?
• Understand the business problem
• Explore the data and become familiar with it.
• Prepare the data for modeling by detecting outliers, treating missing values, transforming variables, etc.
• After data preparation, start running the model, analyze the result and tweak the approach. This is an iterative step till the best possible outcome is achieved.
• Validate the model using a new data set.
• Start implementing the model and track the result to analyse the performance of the model over the period of time.
30) How can you iterate over a list and also retrieve element indices at the same time?
This can be done using the enumerate function which takes every element in a sequence just like in a list and adds its location just before it.
31) During analysis, how do you treat missing values? (get sample code here)
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.There are various factors to be considered when answering this question
32) Explain about the box cox transformation in regression models.
For some reason or the other, the response 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 nonmornla 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.
33) Can you use machine learning for time series analysis?
Yes, it can be used but it depends on the applications.
34) Write a function that takes in two sorted lists and outputs a sorted list that is their union.
First solution which will come to your mind is to merge two lists and short them afterwards
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Python code
def return_union(list_a, list_b):
return sorted(list_a + list_b)
R code
return_union < function(list_a, list_b)
{
list_c<list(c(unlist(list_a),unlist(list_b)))
return(list(list_c[[1]][order(list_c[[1]])]))
}
Generally, the tricky part of the question is not to use any sorting or ordering function. In that case you will have to write your own logic to answer the question and impress your interviewer.
Python code
def return_union(list_a, list_b):
len1 = len(list_a)
len2 = len(list_b)
final_sorted_list = []
j = 0
k = 0
for i in range(len1+len2):
if k == len1:
final_sorted_list.extend(list_b[j:])
break
elif j == len2:
final_sorted_list.extend(list_a[k:])
break
elif list_a[k] < list_b[j]:
final_sorted_list.append(list_a[k])
k += 1
else:
final_sorted_list.append(list_b[j])
j += 1
return final_sorted_list
Similar function can be returned in R as well by following the similar steps.
return_union < function(list_a,list_b)
{
#Initializing length variables
len_a < length(list_a)
len_b < length(list_b)
len < len_a + len_b
#initializing counter variables
j=1
k=1
#Creating an empty list which has length equal to sum of both the lists
list_c < list(rep(NA,len))
#Here goes our for loop
for(i in 1:len)
{
if(j>len_a)
{
list_c[i:len] < list_b[k:len_b]
break
}
else if(k>len_b)
{
list_c[i:len] < list_a[j:len_a]
break
}
else if(list_a[[j]] <= list_b[[k]])
{
list_c[[i]] < list_a[[j]]
j < j+1
}
else if(list_a[[j]] > list_b[[k]])
{
list_c[[i]] < list_b[[k]]
k < k+1
}
}
return(list(unlist(list_c)))
}
35) What is the difference between Bayesian Estimate and Maximum Likelihood Estimation (MLE)?
In bayesian estimate we have some knowledge about the data/problem (prior) .There may be several values of the parameters which explain data and hence we can look for multiple parameters like 5 gammas and 5 lambdas that do this. As a result of Bayesian Estimate, we get multiple models for making multiple predcitions i.e. one for each pair of parameters but with the same prior. So, if a new example need to be predicted than computing the weighted sum of these predictions serves the purpose.
Maximum likelihood does not take prior into consideration (ignores the prior) so it is like being a Bayesian while using some kind of a flat prior.
36) What is Regularization and what kind of problems does regularization solve?
37) How will you tackle an exploding gradient problem?
38) How will you tackle a vanishing gradient problem?
39) How do you decide whether your linear regression model fits the data?
40) What is the difference between squared error and absolute error?
41) What is Machine Learning?
The simplest way to answer this question is – we give the data and equation to the machine. Ask the machine to look at the data and identify the coefficient values in an equation.
For example for the linear regression y=mx+c, we give the data for the variable x, y and the machine learns about the values of m and c from the data.
Recommended Reading: Deep Learning Interview Questions and Answers
42) How are confidence intervals constructed and how will you interpret them?
43) If the training loss of your model is high and almost equal to the validation loss, what does it mean? What should you do?
44) How can you overcome Overfitting?
45) Differentiate between wide and tall data formats?
46) Is Naïve Bayes bad? If yes, under what aspects.
47) How would you develop a model to identify plagiarism?
48) How will you define the number of clusters in a clustering algorithm?
Though the Clustering Algorithm is not specified, this question will mostly be asked in reference to KMeans clustering where “K” defines the number of clusters. The objective of clustering is to group similar entities in a way that the entities within a group are similar to each other but the groups are different from each other.
For example, the following image shows three different groups.
Within Sum of squares is generally used to explain the homogeneity within a cluster. If you plot WSS for a range of number of clusters, you will get the plot shown below. The Graph is generally known as Elbow Curve.
Red circled point in above graph i.e. Number of Cluster =6 is the point after which you don’t see any decrement in WSS. This point is known as bending point and taken as K in K – Means.
This is the widely used approach but few data scientists also use Hierarchical clustering first to create dendograms and identify the distinct groups from there.
49) How important it is to introduce nonlinearities in a neural network and why?
50) Is it possible to perform logistic regression with Microsoft Excel?
It is possible to perform logistic regression with Microsoft Excel. There are two ways to do it using Excel.
a) One is to use Addins provided by many websites which we can use.
b) Second is to use fundamentals of logistic regression and use Excel’s computational power to build a logistic regression
But when this question is being asked in an interview, interviewer is not looking for a name of Addins rather a method using the base excel functionalities.
Let’s use a sample data to learn about logistic regression using Excel. (Example assumes that you are familiar with basic concepts of logistic regression)
Data shown above consists of three variables where X1 and X2 are independent variables and Y is a class variable. We have kept only 2 categories for our purpose of binary logistic regression classifier.
Next we have to create a logit function using independent variables, i.e.
Logit = L = β_{0} + β_{1}*X_{1} + β_{2}*X_{2}
We have kept the initial values of beta 1, beta 2 as 0.1 for now and we will use Excel Solve to optimize the beta values in order to maximize our log likelihood estimate.
Assuming that you are aware of logistic regression basics, we calculate probability values from Logit using following formula:
Probability= e^Logit/(1+ e^Logit )
e is base of natural logarithm i.e. e = 2.71828163
Let’s put it into excel formula to calculate probability values for each of the observation.
The conditional probability is the probability of Predicted Y, given set of independent variables X.
And this p can be calculated as
P〖(X)〗^Yactual*[1P〖(X)〗^(1Yactual)]
Then we have to take natural log of the above function
ln〖[ 〗 P〖(X)〗^Yactual*[1P(X)^(1Yactual) ]]
Which turns out to be –
Yactual*ln〖[ 〗 P(X)]*(Yactual 1)*ln[1P(X)]
Log likelihood function LL is the sum of above equation for all the observations
Log likelihood LL will be sum of column G, which we just calculated
The objective is to maximize the Log Likelihood i.e. cell H2 in this example. We have to maximize H2 by optimizing B_{0}, B_{1}, and B_{2.}
We’ll use Excel’s solver addin to achieve the same.
Excel comes with this Addin preinstalled and you must see it under Data Tab in Excel as shown below
If you don’t see it there then make sure if you have loaded it. To load an addin in Excel,
Go to File >> Options >> AddIns and see if checkbox in front of required addin is checked or not? Make sure to check it to load an addin into Excel.
If you don’t see Solver Addin there, go to the bottom of the screen (Manage AddIns) and click on OK. Next you will see a popup window which should have your Solver addin present. Check the checkbox infront of the addin name. If you don’t see it there as well click on browse and direct it to the required folder which contains Solver AddIn.
Once you have your Solver loaded, click on Solver icon under Data tab and You will see a new window popped up like –
Put H2 in set objective, select max and fill cells E2 to E4 in next form field.
By doing this we have told Solver to Maximize H2 by changing values in cells E2 to E4.
Now click on Solve button at the bottom –
You will see a popup like below 
This shows that Solver has found a local maxima solution but we are in need of Global Maxima Output. Keep clicking on Continue until it shows the below popup
It shows that Solver was able to find and converge the solution. In case it is not able to converge it will throw an error. Select “Keep Solver Solution” and Click on OK to accept the solution provided by Solver.
Now, you can see that value of Beta coefficients from B_{0,} B_{1} B_{2 }have changed and our Log Likelihood function has been maximized.
Using these values of Betas you can calculate the probability and hence response variable by deciding the probability cutoff.
51) What do you understand by Fuzzy merging ? Which language will you use to handle it?
52) What is the difference between skewed and uniform distribution?
When the observations in a dataset are spread equally across the range of distribution, then it is referred to as uniform distribution. There are no clear perks in an uniform distribution. Distributions that have more observations on one side of the graph than the other are referred to as skewed distribution.Distributions with fewer observations on the left ( towards lower values) are said to be skewed left and distributions with fewer observation on the right ( towards higher values) are said to be skewed right.
53) You created a predictive model of a quantitative outcome variable using multiple regressions. What are the steps you would follow to validate the model?
Since the question asked, is about post model building exercise, we will assume that you have already tested for null hypothesis, multi collinearity and Standard error of coefficients.
Once you have built the model, you should check for following –
· Global Ftest to see the significance of group of independent variables on dependent variable
· R^2
· Adjusted R^2
· RMSE, MAPE
In addition to above mentioned quantitative metrics you should also check for
· Residual plot
· Assumptions of linear regression
54) What do you understand by Hypothesis in the content of Machine Learning?
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55) What do you understand by Recall and Precision?
Recall measures "Of all the actual true samples how many did we classify as true?"
Precision measures "Of all the samples we classified as true how many are actually true?"
We will explain this with a simple example for better understanding 
Imagine that your wife gave you surprises every year on your anniversary in last 12 years. One day all of a sudden your wife asks "Darling, do you remember all anniversary surprises from me?".
This simple question puts your life into danger.To save your life, you need to Recall all 12 anniversary surprises from your memory. Thus, Recall(R) is the ratio of number of events you can correctly recall to the number of all correct events. If you can recall all the 12 surprises correctly then the recall ratio is 1 (100%) but if you can recall only 10 suprises correctly of the 12 then the recall ratio is 0.83 (83.3%).
However , you might be wrong in some cases. For instance, you answer 15 times, 10 times the surprises you guess are correct and 5 wrong. This implies that your recall ratio is 100% but the precision is 66.67%.
Precision is the ratio of number of events you can correctly recall to a number of all events you recall (combination of wrong and correct recalls).
56) How will you find the right K for Kmeans?
57) Why L1 regularizations causes parameter sparsity whereas L2 regularization does not?
Regularizations in statistics or in the field of machine learning is used to include some extra information in order to solve a problem in a better way. L1 & L2 regularizations are generally used to add constraints to optimization problems.
In the example shown above H_{0} is a hypothesis. If you observe, in L_{1} there is a high likelihood to hit the corners as solutions while in L_{2}, it doesn’t. So in L_{1} variables are penalized more as compared to L2 which results into sparsity.
In other words, errors are squared in L_{2}, so model sees higher error and tries to minimize that squared error.
58) How can you deal with different types of seasonality in time series modelling? (get 100+ solved code examples here)
Seasonality in time series occurs when time series shows a repeated pattern over time. E.g., stationary sales decreases during holiday season, air conditioner sales increases during the summers etc. are few examples of seasonality in a time series.
Seasonality makes your time series nonstationary because average value of the variables at different time periods. Differentiating a time series is generally known as the best method of removing seasonality from a time series. Seasonal differencing can be defined as a numerical difference between a particular value and a value with a periodic lag (i.e. 12, if monthly seasonality is present)
59) Explain evaluation protocols for testing your models? Compare holdout vs kfold cross validation vs iterated kfold crossvalidation methods of testing.
60) What do you understand by conjugateprior with respect to Naïve Bayes?
61) Can you cite some examples where a false positive is important than a false negative?
Before we start, let us understand what are false positives and what are false negatives.
False Positives are the cases where you wrongly classified a nonevent as an event a.k.a Type I error.
And, False Negatives are the cases where you wrongly classify events as nonevents, a.k.a Type II error.
In medical field, assume you have to give chemo therapy to patients. Your lab tests patients for certain vital information and based on those results they decide to give radiation therapy to a patient.
Assume a patient comes to that hospital and he is tested positive for cancer (But he doesn’t have cancer) based on lab prediction. What will happen to him? (Assuming Sensitivity is 1)
One more example might come from marketing. Let’s say an ecommerce company decided to give $1000 Gift voucher to the customers whom they assume to purchase at least $5000 worth of items. They send free voucher mail directly to 100 customers without any minimum purchase condition because they assume to make at least 20% profit on sold items above 5K.
Now what if they have sent it to false positive cases?
62) Can you cite some examples where a false negative important than a false positive? (get 100+ solved code examples here)
Assume there is an airport ‘A’ which has received high security threats and based on certain characteristics they identify whether a particular passenger can be a threat or not. Due to shortage of staff they decided to scan passenger being predicted as risk positives by their predictive model.
What will happen if a true threat customer is being flagged as nonthreat by airport model?
Another example can be judicial system. What if Jury or judge decide to make a criminal go free?
What if you rejected to marry a very good person based on your predictive model and you happen to meet him/her after few years and realize that you had a false negative?
63) Can you cite some examples where both false positive and false negatives are equally important?
In the banking industry giving loans is the primary source of making money but at the same time if your repayment rate is not good you will not make any profit, rather you will risk huge losses.
Banks don’t want to lose good customers and at the same point of time they don’t want to acquire bad customers. In this scenario both the false positives and false negatives become very important to measure.
These days we hear many cases of players using steroids during sport competitions Every player has to go through a steroid test before the game starts. A false positive can ruin the career of a Great sportsman and a false negative can make the game unfair.
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64) Can you explain the difference between a Test Set and a Validation Set?
Validation set can be considered as a part of the training set as it is used for parameter selection and to avoid Overfitting of the model being built. On the other hand, test set is used for testing or evaluating the performance of a trained machine leaning model.
In simple terms ,the differences can be summarized as
65) What makes a dataset gold standard?
66) 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, RF 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 senstivity is pretty straight forward
Senstivity = True Positives /Positives in Actual Dependent Variable
Where, True positives are Positive events which are correctly classified as Positives.
67) What is the importance of having a selection bias? (get 100+ solved code examples here)
Selection Bias occurs when there is no appropriate randomization achieved while selecting individuals, groups or data to be analyzed. Selection bias implies that the obtained sample does not exactly represent the population that was actually intended to be analyzed. Selection bias consists of Sampling Bias, Data, Attribute, and Time Interval.
68) Given that you let the models run long enough, will all gradient descent algorithms lead to the same model when working with Logistic or Linear regression problems?
69) Differentiate between Batch Gradient Descent, MiniBatch Gradient Descent, and Stochastic Gradient Descent.
70) How do data management procedures like missing data handling make selection bias worse?
Missing value treatment is one of the primary tasks which a data scientist is supposed to do before starting data analysis. There are multiple methods for missing value treatment. If not done properly, it could potentially result in selection bias. Let see few missing value treatment examples and their impact on selection
Complete Case Treatment: Complete case treatment is when you remove an entire row in data even if one value is missing. You could achieve a selection bias if your values are not missing at random and they have some pattern. Assume you are conducting a survey and few people didn’t specify their gender. Would you remove all those people? Can’t it tell a different story?
Available case analysis: Let say you are trying to calculate a correlation matrix for data so you might remove the missing values from variables that are needed for that particular correlation coefficient. In this case, your values will not be fully correct as they are coming from population sets.
Mean Substitution: In this method, missing values are replaced with the mean of other available values. This might make your distribution biased e.g., standard deviation, correlation and regression are mostly dependent on the mean value of variables.
Hence, various data management procedures might include selection bias in your data if not chosen correctly.
71) What are the advantages and disadvantages of using regularization methods like Ridge Regression?
72) What do you understand by long and wide data formats?
73) What do you understand by outliers and inliers? What would you do if you find them in your dataset?
74) Write a program in Python that takes input as the diameter of a coin and weight of the coin and produces output as the money value of the coin.
75) What are the basic assumptions to be made for linear regression? (get sample code here)
Normality of error distribution, statistical independence of errors, linearity and additivity.
76) Can you write the formula to calculate Rsquare?
RSquare can be calculated using the below formular 
1  (Residual Sum of Squares/ Total Sum of Squares)
77) What is the advantage of performing dimensionality reduction before fitting an SVM?
Support Vector Machine Learning Algorithm performs better in the reduced space. It is beneficial to perform dimensionality reduction before fitting an SVM if the number of features is large when compared to the number of observations.
78) How will you assess the statistical significance of an insight whether it is a real insight or just by chance?
Statistical importance of an insight can be accessed using Hypothesis Testing.
79) How would you create a taxonomy to identify key customer trends in unstructured data?
The best way to approach this question is to mention that it is good to check with the business owner and understand their objectives before categorizing the data. Having done this, it is always good to follow an iterative approach by pulling new data samples and improving the model accordingly by validating it for accuracy by soliciting feedback from the stakeholders of the business. This helps ensure that your model is producing actionable results and improving over the time.
80) How will you find the correlation between a categorical variable and a continuous variable ?
You can use the analysis of covariance technqiue to find the correlation between a categorical variable and a continuous variable.
81) Is it better to have too many false negatives or too many false positives?
82) In experimental design, is it necessary to do randomization? If yes, why?
83) What are the benefits of using a convolutional neural network over a fully connected network when working with image classification problems?
84) What are the benefits of using a recurrent neural network over a fully connected network when working with text data?
85) What do you understand by feature vectors?
86) How can outlier values be treated?
Outlier values can be identified by using univariate or any other graphical analysis method. If the number of outlier values is few then they can be assessed individually but for a large number of outliers, the values can be substituted with either the 99th or the 1st percentile values. All extreme values are not outlier values. The most common ways to treat outlier values –
1) To change the value and bring in within a range
2) To just remove the value.
87) How can you assess a good logistic model?
There are various methods to assess the results of logistic regression analysis
• Using Classification Matrix to look at the true negatives and false positives.
• Concordance that helps identify the ability of the logistic model to differentiate between the event happening and not happening.
• Lift helps assess the logistic model by comparing it with random selection.
88) What are categorical variables?
89) What is the benefit of weight initialization in neural networks?
90) How does the use of dropout work as a regularizer for deep neural networks?
91) How beneficial is dropout regularization in deep learning models? Does it speed up or slow down the training process and why?
92) How will you explain logistic regression to an economist, physicianscientist, and biologist?
93) What is the benefit of batch normalization?
94) Give some situations where you will use an SVM over a RandomForest Machine Learning algorithm and viceversa.
95) What is multicollinearity and how you can overcome it?
SVM and Random Forest are both used in classification problems.
a) If you are sure that your data is outlier free and clean then go for SVM. It is the opposite  if your data might contain outliers then Random forest would be the best choice
b) Generally, SVM consumes more computational power than Random Forest, so if you are constrained with memory go for the Random Forest machine learning algorithm.
c) Random Forest gives you a very good idea of variable importance in your data, so if you want to have variable importance then choose the Random Forest machine learning algorithm.
d) Random Forest machine learning algorithms are preferred for multiclass problems.
e) SVM is preferred in multidimensional problem set  like text classification
but as a good data scientist, you should experiment with both of them and test for accuracy, or rather you can use an ensemble of many Machine Learning techniques.
96) What is the curse of dimensionality?
97) Do you need to do feature engineering and feature extraction when applying deep learning models?
98) How will you calculate the accuracy of a model using a confusion matrix?
99) According to the universal approximation theorem, any function can be approximated as closely as required using single collinearity. Then why people use more?
100) Explain the use of Combinatorics in data science.
101) You are given a dataset with 1500 observations and 15 features. How many observations you will select in each decision tree in a random forest?
Each decision tree has a subset of features but includes all the observations from the dataset. In this case, the answer will be 1500 as the tree will include all the observations from the dataset.
102) How will you evaluate the performance of a logistic regression model?
It's very much obvious that you would mention accuracy as the answer to this question but since logistic regression is not the same as linear regression it will mislead. You should mention how you will use the confusion matrix to evaluate the performance and the various statistics related to it like Precision, Specificity, Sensitivity, or Recall. You get bonus points for mentioning Concordance, Discordance, and AUC.
Data Science PuzzlesBrain Storming/ Puzzle based Data Science Interview Questions asked in Data Scientist Job Interviews
1) How many Piano Tuners are there in Chicago?
To solve this kind of problem, we need to know –
Can you tell if the equation given below is linear or not?
Emp_sal= 2000+2.5(emp_age)^{2}
Yes it is a linear equation as the coefficients are linear.
What will be the output of the following R programming code ?
var2< c("I","Love,"ProjectPro")
var2
It will give an error.
How many Pianos are there in Chicago?
How often would a Piano require tuning?
How much time does it take for each tuning?
We need to build these estimates to solve this kind of a problem. Suppose, let’s assume Chicago has close to 10 million people and on an average there are 2 people in a house. For every 20 households there is 1 Piano. Now the question how many pianos are there can be answered. 1 in 20 households has a piano, so approximately 250,000 pianos are there in Chicago.
Now the next question is“How often would a Piano require tuning? There is no exact answer to this question. It could be once a year or twice a year. You need to approach this question as the interviewer is trying to test your knowledge on whether you take this into consideration or not. Let’s suppose each piano requires tuning once a year so on the whole 250,000 piano tunings are required.
Let’s suppose that a piano tuner works for 50 weeks in a year considering a 5 day week. Thus a piano tuner works for 250 days in a year. Let’s suppose tuning a piano takes 2 hours then in an 8 hour workday the piano tuner would be able to tune only 4 pianos. Considering this rate, a piano tuner can tune 1000 pianos a year.
Thus, 250 piano tuners are required in Chicago considering the above estimates.
2) There is a race track with five lanes. There are 25 horses of which you want to find out the three fastest horses. What is the minimal number of races needed to identify the 3 fastest horses of those 25?
Divide the 25 horses into 5 groups where each group contains 5 horses. Race between all the 5 groups (5 races) will determine the winners of each group. A race between all the winners will determine the winner of the winners and must be the fastest horse. A final race between the 2^{nd} and 3^{rd} place from the winners group along with the 1^{st} and 2^{nd} place of thee second place group along with the third place horse will determine the second and third fastest horse from the group of 25.
3) Estimate the number of french fries sold by McDonald's everyday.
4) How many times in a day does a clock’s hand overlap?
5) You have two beakers. The first beaker contains 4 litre of water and the second one contains 5 litres of water.How can you our exactly 7 litres of water into a bucket?
6) A coin is flipped 1000 times and 560 times heads show up. Do you think the coin is biased?
7) Estimate the number of tennis balls that can fit into a plane.
8) How many haircuts do you think happen in US every year?
9) In a city where residents prefer only boys, every family in the city continues to give birth to children until a boy is born. If a girl is born, they plan for another child. If a boy is born, they stop. Find out the proportion of boys to girls in the city.
1. Explain the central limit theorem.
2. What is the relevance of the central limit theorem to a class of freshmen in the social sciences who hardly have any knowledge about statistics?
3. Given a dataset, show me how Euclidean Distance works in three dimensions.
4. How will you prevent overfitting when creating a statistical model?
1) Explain about the range function.
2) How can you freeze an already built machine learning model for later use ? What is the command you would use?
3) Differentiate between func anf func()
4) Write the command to import a decision tree classification algorithm using sklearn library.
5) What do you understand by pickling in Python?
How can you ensure that you don’t analyze something that ends up producing meaningless results?
 Gaganpreet Singh, Data Scientist
So, there you have over 120 data science interview questions and answers for most of them too. These are some of the more common interview questions for data scientists around data, statistics, and data science that can be asked in the interviews. We will come up with more questions – specific to language, Python/ R, in the subsequent articles, and fulfill our goal of providing 120 data science interview questions PDF with answers to our readers.
• Keep on adding technical skills to your data scientist’s toolbox.
• Improve your scientific axiom
• Learn the language of business as the insights from a data scientist help in reshaping the entire organization.
The important tip, to nail a data science interview is to be confident with the answers without bluffing. If you are wellversed with a particular technology whether it is Python, R, Hadoop, Spark or any other big data technology ensure that you can back this up but if you are not strong in a particular area do not mention it unless asked about it. The above list of data scientist job interview questions is not an exhaustive one. Every company has a different approach to interviewing data scientists. However, we do hope that the above data science technical interview questions elucidate the data science interview process and provide an understanding of the type of data scientist job interview questions asked when companies are hiring data people.
We request industry experts and data scientists to chime in their suggestions in comments for openended data science interview questions to help students understand the best way to approach the interviewer and help them nail the interview.
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