Every Data Scientist should be skilful enough in several fields like statistics, data analytics, and visualisation techniques, machine learning & deep learning. Hence, this is not an easy field for anyone, but here are the top data science interview questions and answers for all levels that can help you prepare thoroughly.
In this post, we have mentioned 100 data science interview questions that are frequently asked in the interview for data sciencerelated job roles. Here, we have also included some ethical questions, to boost the interview preparation overall.
Table of Contents
The study of data science involves applying scientific techniques, problemsolving, implementing different data structure algorithms, building machine learning models, and statistical analysis using languages such as R and Python. All these methods have one aim, which is to extract knowledge and insights from both structured and unstructured data. Python libraries such as Pandas, NumPy, Matplotlib, Scikit Learn, etc., play a significant role in data interpretation.
Several techniques mitigate overfitting in the model, such as crossvalidation, regularisation methods (such as L1 and L2 regularisation), and pruning (for decision trees). These techniques also work when we use a simpler model with fewer parameters.
Crossvalidation includes splitting the data into multiple subsets to get the performance of the model. Here, we need to train the model on k1 parts and test on the remaining part.
This process is repeated k times, each time with a different test part. Finally, the obtained results are averaged to get a new estimated performance of the model and better generalisation of the new data.
A table that is used to evaluate any classification model’s performance and gives a final score to find the most optimum model. The matrix consists of the actual classifications vs predicted outcomes with four key values, shown below.
CONFUSION MATRIX  Predicted Positive  Predicted Negative 
Actual Positive  True Positive (TP)  False Negative (FN) 
Actual Negative  False Positive (FP)  True Negative (TN) 
Plotting a Receiver Operating Characteristic (ROC) curve is done to measure the performance of a binary classification model. This is the graph of the true positive rate vs the false positive rate. The higher area under the curve represents its higher ability to solve the question or the model is said to be good in classification.
For better comparison (compact dynamic input) and to avoid huge values in matrix multiplication, the sigmoid function is used. It compresses the data between 0 and 1. This Sshaped function is given as:
A decision tree is one of the supervised machine learning algorithms that can be implemented for regression and classification. The tree works recursively, splitting the data into subsets and assigning the values in trees according to their input values. The following is the demonstration of the decision tree
Feature  Supervised Learning  Unsupervised Learning 
Definition  Learning with labelled data  Learning with unlabeled data 
Aim  Classify or predict data  Make clusters or find hidden patterns in the data 
Input Data  Input data with known output labels  Data input without any labels 
Outcome  Predictive model  Grouped or Structured clustering 
Examples  Classification, Regression  Clustering, Association 
Algorithms  Decision Trees, Linear Regression, SVM  Kmeans, Hierarchical Clustering, PCA 
Evaluation  Accuracy, Precision, Recall, F1score  Inertia, Cluster Purity, Silhouette Score 
The pvalue (probability value) is used to assume the null hypothesis is true. When we find a high pvalue, our hypothesis is true. Finding a low pvalue means there is strong evidence that the null hypothesis is false.
Feature  Box Plot  Whisker Plot 
Components  Box (interquartile range), median line, whiskers (min/max within 1.5 IQR), outliers  Similar to the box plot; highlights whiskers and range 
Focus  Summary statistics (median, quartiles, outliers)  Data spread and range 
Usage  Analysing data distribution and identifying outliers  Visualising data spread and detecting variability 
The structured data is stored in predetermined formats such as databases or labelled searchable data, thus it is organised. Unstructured data lacks any labelled entity or predetermined format, which includes images, text, videos, etc. hence it is hard to analyse.
It is a theorem that for large enough samples, the distribution of the sample mean on a graph will be a normal distribution, no matter how it is distributed.
Feature  Histogram  Bar Chart 
Data Type  Continuous data  Categorical Data 
Bars  Adjacent bars touch  Bars are separated by spaces 
XAxis  Represents intervals or bins of continuous data  Represents categories

Purpose  Shows the distribution of numerical data  Compares different categories 
Bias occurs due to errors in training data (underfitting), while variance is the variation in the output of training and testing data generated by a model (overfitting). The tradeoff involves finding a model complexity that simplifies both bias and variance and finding the best solution on unseen data.
The values we obtain from a confusion matrix are true positive (TP), true negative (TN), false positive (FP), and false negative (FN). The formulas to calculate are:
The strength of ensemble learning comes from aggregating many similar models together to reduce errors and increase robustness compared with individual methods. Some examples include Bagging, Boasting, and Stacking.
Feature  Bagging  Boasting 
Base Learners  It can train multiple independent models in parallel  Trains multiple models subsequently each correcting errors of the previous ones. 
Model Complexity  It uses highvariance and lowbias models  It often uses simple models like a decision tree to reduce overfitting 
Weighting of models  All models are weighted equally  Models are weighted based on their performance with more emphasis on correcting the errors 
Output Combination  Aggregates predictions through averaging or voting  Combines predictions using a weighted sum or boosting algorithm 
Examples  Random Forest  Gradient Boosting, AdaBoost 
With the help of regularisation, we can add a penalty to any model’s capacity so that it can prevent the model from fitting the noise in training data. This can be done by adding a regularisation term in the loss function. Hence, we can prevent overfitting in the ML model.
Feature  L1 Regularization (Lasso)  L2 Regularization (Ridge) 
Penalty Terms  Shrinks coefficients but rarely sets them to zero  Shrinks coefficients but rarely sets them to zero 
Effect on Coefficients  Encourages sparsity; some coefficients become exactly zero  Shrinks coefficients but rarely sets them to zero

Feature Selection  Performs feature selection by eliminating irrelevant features  Retains all features but reduces their impact 
Use Case  Useful when having many irrelevant features  Useful for avoiding overfitting with many features 
Feature engineering is the process of designing new features, or modifying existing ones, so as to make existing models easier to use, more generalizable, and more accurate when predicting future outcomes.
Data Augmentation: By applying new transformations on the matrix, like rotation, scaling, etc., new data samples are created. This method is commonly used in text processing to increase data diversity.
Data Synthesis: This method includes generating new data samples by generative models like GANs or VAEs. It’s used when there is insufficient data, hence creating realistic dynamic data.
Parametric frameworks rely on rigidly defined rules for the relationship between variables, featuring a set quantity of parameters. Conversely, nonparametric frameworks adjust the number of parameters based on the intricacy of the data without committing to a particular structure.
Principal Component Analysis (PCA) helps to minimise the dataset’s complexity by recognizing the principal components, which are responsible for the highest level of variation within the data.
This method is used highly in areas like data visualisation, especially in data compression, noise minimization, and feature extraction. Features like images and image analysis, biological informatics are extracted.
An example of a supervised learning technique that has gained wide usage is the Support Vector Machine (SVM). This algorithm can be used within both the classification and regression boundaries. SVM finds the hyperplane that most constructively divides various classes within the feature space. It works as:
One of the most used algorithms in unsupervised learning is kmeans clustering because it can categorise data into clusters by comparing similarities between data points. It can cluster all observations into k groups using the cluster’s mean average.
kmeans clustering easily facilitates applications like customer segmentation, genetic clustering, anomaly detection, etc.
Covariance shows the connection between the variables. To show the nature of variables, if the covariance is high, that means the variables move in the same direction, i.e. rising or falling together.
If one of the variables goes up and the other goes down, the covariance will be negative. Covariance is also a useful measure in portfolio theory and risk management.
Feature  KMeans Clustering  Hierarchical Clustering 
No. of Clusters  Requires number of clusters (k)  Does not require a prespecified number of clusters 
Cluster Formation  Outputs k clusters based on centroids  Output clusters based on the distance between data points 
Cluster Hierarchy  Does not provide a hierarchy  Produce a hierarchy of clusters in a treelike structure 
Complexity  Faster and more scalable for large datasets  Slower and less scalable for large datasets 
Result  May produce different results each run due to random initialization  Produces more stable results because of hierarchy 
Example  Customer Segmentation based on purchase behaviour  Biological Taxonomy based on genetic similarities 
Batch Gradient Descent calculates the gradient by using the whole dataset, which is appropriate for datasets that are smaller but take more time. On the other hand, Stochastic Gradient Descent calculates the gradient by focusing on a single data point, making it quicker for bigger datasets but introducing more variability.
LDA (Latent Dirichlet Allocation) is a technique to categorise the given text into a document by assuming documents are like a mixture of topics, and the topics are a combination of words. Hence, it optimises all the distributions via an iterative process by this assumption.
Some of the methods for optimising hyperparameters are:
One of the initial tasks in Natural Language Processing (NLP) is “word embedding” which represents words as points in vector space. It captures the meaningful connections among words by considering their context within a collection of texts. For determining the sentiment of text, document categorising and also in text translation, word embeddings are applied. This also helps in increasing the effectiveness of several models.
Autoencoders are unsupervised neural networks in which encoders and decoders are able to create a new version of the same data for the reason that it can convert an input that is close to an output.
Hence, an autoencoder can be trained to gain as much information as possible from the original data and encode them into the most reduced form possible, preserving its main key components. That is how input is dimensionally reduced into simpler forms and lowers the complexity of the input.
Here are the methods mentioned to identify anomalies or datasets that are different from the majority of the dataset:
Feature  Batch Processing  Stream Processing 
Data Handling  Processes large blocks of data at scheduled intervals.  Processes data continuously in real time. 
Latency  Higher latency; results are available after batch completion.  Low latency; results are available almost immediately. 
Use Cases  Suitable for endofday reports, backups, and ETL processes.  Suitable for realtime analytics, monitoring, and alerting. 
Resource Usage  Often requires significant resources at specific times.  Spreads resource usage more evenly over time. 
Complexity  Simpler to implement and manage.  More complex, requiring handling of realtime data streams. 
Examples  Hadoop, Apache Spark (Batch mode).  Apache Kafka, Apache Flink, Apache Storm. 
In machine learning, kernel methods employ kernel functions to subtly transform data into more complex dimensions without directly computing the transformed feature vectors. This capability enables linear algorithms to represent nonlinear patterns within the data.
The transformed feature vector, however, is simply the dot product of two transformed feature vectors computed in the higherdimensional space. By these means, we can construct arbitrarily complicated decision boundaries in the input space.
Feature  Generative Model  Discriminative Model 
Objective  Learn the joint probability distribution of the input features and the labels.  Learn the conditional probability of the labels given the input features. 
Output  It can generate new data similar to the training data.  Only predicts the labels for the input data. 
Complexity  Typically more complex, as they model the entire distribution.  Generally simpler, as they focus on modelling the decision boundary. 
Bayesian inference is a mathematical method of statistical reasoning that uses an application of Bayes’ theorem to update the probability of a hypothesis in new data. In the context of machine learning, Bayesian inference is used to determine parameter values in models, make predictions, and estimate the uncertainty of those predictions.
Batch Normalisation:
Layer Normalisation:
Generative Adversarial Networks (GANs) are made up of two interconnected neural networks: a creator and an evaluator. The creator produces artificial data, and the evaluator can tell the difference between genuine and artificial data.
They undergo training in a competitive manner, with the creator improving its ability to produce more lifelike data to deceive the evaluator and the evaluator enhancing its skill in identifying genuine from artificial data. This rivalry results in the creator being able to produce data that closely resembles real data, thereby creating new data samples.
You can retrain the model on a different dataset, tuning the insights you already have based on learning one task towards another, and you can ‘reinforce’ your task via fewer training hours and less data.
Feature  Parametric Model  NonParametric Model 
Assumptions  Assumes a specific functional form of the relationship.  Does not assume a specific functional form. 
Number of Parameters  Has a fixed number of parameters.  Has a flexible number of parameters. 
Parameter Learning  Parameters are learned from the data.  Parameters grow with the amount of data. 
Examples  Linear and Logistic Regression.  Decision Trees, kNearest Neighbors, SVM with RBF Kernel. 
As a strategy to ease working with highdimensional data, feature hashing (also known as the hashing trick) reduces the dimensions of data by turning them into a feature vector of fixed length. It uses a hash function that transforms the original features into a smaller set of hash values.
This approach facilitates easier storage and processing, particularly when handling extensive datasets containing numerous features. Nevertheless, this can lead to collisions between regions of feature space in which various features have the same hash value potentially affecting the model performance.
Advantages:
Disadvantages:
Suppose your dataset has more features than observations. In that case, you can employ methods such as feature selection, dimensionality reduction (like PCA), or regularisation (such as L1 regularisation) to decrease the feature space and avoid overfitting. This contributes to enhancing model effectiveness and understandability.
To handle a dataset with both numerical and categorical variables, we first need to clean the data, encode the variables using onehot encoding, and normalise all numerical values. We then take an ML model that supports mixed data types, such as random forests or decision trees.
The decrease in the performance of an ML model over time due to changes in data distribution is called model drift. It can be detected by metrics to perform model efficiency outcomes and more statistical tests on those data distributions. Comparing actual outcomes using several drift detection methods aids in validation checks.
The main purpose of stratified sampling is to ensure that each subgroup within a given population in a dataset is sufficient. It is used to maintain the proportion of different categories or classes within the dataset for a balanced representation.
Performing sentiment analysis is one of the most important parts of knowing the nature of customer reviews. Preprocessing the data by converting normal text to numerical features using methods like word embeddings or TFIDF extracts all relations in vectors. And then applying a classification model like Naive Bayes to those vectors to know if the sentiments are positive, negative, or neutral.
In Fourier transformation, a signal is decomposed into its constituent frequencies, its most calculative format. Using that, we can do the analysis of its frequency components using data science models. It is used in signal denoising, feature extraction in timeseries data, and also in transforming data into frequency for analysis and modelling.
You could implement these methods:
Following are the techniques to handle imbalanced classes.
For this type of query, you can delete rows or columns with missing values, but it could not work all the time. Rather, finding missing values with mean, median, or mode and also include KNN imputation to estimate missing values.
Moreover, filling the missing values based on the last known value (forward fill) or the next known value (backward fill) will deal with this situation. Depending upon the task and model, we could put different estimates to refill the empty spaces with assumptions.
Some challenges with big data include scalability, storage, and processing speed. Address these by using distributed computing frameworks (like Hadoop or Spark), data partitioning, and efficient algorithms.
You can add any of your ML projects for this. But make sure to follow a format: first, give a short overview of the project, then assess the problems you faced, like imbalanced data or missing data and feature engineering, and then answer the solution to those problems.
Ethical considerations in data science include privacy, consent, and bias. To ensure fairness, use representative datasets, regularly audit models for bias, and involve diverse teams in model development.
In many cases, organisations prefer to receive preprocessed data to ensure consistency and accuracy. However, the level of processing required can vary based on the organisation’s specific needs and capabilities. You must generally prefer preprocessing the data to avoid any anomaly that could affect your results significantly.
Taking the source verification, network analysis, and caption text analysis could help initially. After that, these steps could help:
This answer could vary a lot, but you must mention only the trusted sources where you can find datasets. For example, Kaggle and Wiki.
One optimising method could be Stochastic Gradient Descent (SGD) for its efficiency in handling large datasets and Adam for its adaptive learning rate and momentum, which can lead to faster convergence and better performance in many cases.
Consider using Kmeans because of its ease of use and it can handle large amounts of data very well. DBSCAN can also be used for its skill in spotting groups of different forms and dimensions without needing to set a specific number of groups in advance.
You would collect historical electricity consumption data, analyse trends and patterns, engineer relevant features, select a forecasting model like ARIMA or LSTM, train the model, and use it to forecast future electricity consumption for the city.
It is important to prioritise tasks based on their urgency and impact. Divide tasks into smaller sections, establish specific milestones, and utilise project management software to monitor advancement and ensure projects are finished on time.
You can simplify the main points, incorporate visual aids such as charts and graphs, and connect the results to their business implications. Promote inquiries for clarity and adapt explanations according to their input.
The most significant way of this today would be the internet. Tons of research papers and blogs are more than enough to keep up with the latest developments. Also, reading industry journals, social media, conferences, and webinars, enrolling in online courses, and engaging in relevant online communities etc. would also help.
When you have limited information, assess what is available, identify missing key points, and think about how those gaps could affect your decisionmaking. Next, rely on your own intuition, consult with specialists, and make educated guesses using your past expertise and learnings. Ultimately, you observe the results and make changes based on updated information.
Here, collecting appropriate data and displaying it using visuals such as charts or graphs is important. Emphasise the main ideas and how they back the fresh view, acknowledging possible worries and showcasing the advantages of the suggested modification. This aids in constructing a convincing argument supported by evidence.
Frequent communication with the stakeholders will help in gathering any related information regarding ambiguity in a project. Defining goals and task management afterwards is also important. Prioritising tasks and dividing projects into small tasks will help to conquer ambiguity.
Confirmation of the error is the key to starting the fixing process. Here, answer any significant error you find while working on any model, explain how to solve the problem, and indicate whether any team member is affected by/helping with the situation.
The first step is to research and grasp the fundamentals using tutorials and documentation. Next, you utilise this information for minor duties in the project, reaching out to coworkers or internet forums for assistance when necessary. By deconstructing the learning process and maintaining regular practice, you effectively incorporate the new tool into your work routine to meet the project deadline.
To guarantee the reproducibility of analyses and models, it is important to document every step of the workflow, such as data preprocessing, feature engineering, model selection, and evaluation metrics. Git is employed to monitor modifications and cooperate with peers. Sharing code, data, and documentation enables others to replicate your findings.
When juggling several projects, you organise tasks by their deadlines and level of significance. You divide every project into smaller, manageable tasks and either make a schedule or utilise project management tools to monitor advancement. Consistent stakeholder communication is important for setting and meeting project expectations and goals. Deadline handling is the key task here to prevent any loss.
Analysing data can enhance network performance by detecting and fixing bottlenecks, foreseeing equipment breakdowns, and finetuning network setups.
This question is generally asked in Ecommerce organisations or related domains in data science. Here, you can utilise recommendation algorithms such as collaborative filtering or those based on user purchase history and behaviour patterns of similar users.
Risk management is one of the most important roles in finance. It’s a responsible task for both organisations while maintaining data integrity, and several policies should be made through the risk management process. It can be done by categorising and prioritising risks and then evaluating and addressing the required solutions.
Analysing data can improve inventory control by predicting demand, spotting patterns, and optimising pricing and promotions using customer actions
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Machine learning is able to anticipate patient results by examining information on patients, such as their demographics, medical background, and laboratory findings, in order to recognize trends that may suggest specific conditions or results.
Data analytics tools and techniques are used to measure effectiveness through metrics such as:
The analysis of user interaction and feelings towards content can be done through natural language processing (NLP) to determine user engagement and sentiment.
By examining usage patterns, pinpointing areas of inefficiency, and suggesting energysaving measures, data science has the potential to enhance energy consumption efficiency.
Analysing data can enhance traffic flow by examining traffic patterns, pinpointing congestion points, and optimising traffic signal timings.
Early defect detection is the key to enhancing quality control in any manufacturing process. Data analytics can help a lot by using several data processing techniques to monitor any defects early in the process. After addressing the problem, instant solutions, maintenance requirements, and optimization are carried out.
One of the most important things for smoothly running a model is checking for biases at regular intervals. All the data is dynamic to keep the model up to date. But the dynamic data often brings many variations, and hence, sometimes we need to change the data preprocessing methods also. Several methods can be used to detect biases in the model and by this, we can promote fairness and flexibility.
In data science projects, the ownership of the data used can vary depending on the source and context. In most cases, the organisation that receives the data has the right to own it. There are many deeplevel privacy policies regarding data ownership, and one must follow them while sharing data.
Sharing data with other organisations alone brings many data processing procedures. The raw data can harm the integrity and policy of both research organisations. Mutual understanding is also required to avoid any small error conflicts.
To ensure this, all the model deployments should be done responsibly and simultaneously conducting comprehensive tests. Keeping track of how the models are performing establishes a way for use to ensure it does not cause any harm to the organisation.
This can be done through various methods, including:
Yes, this process can be automated using some popular tools like TensorBoard, Google Cloud AI platform, Databricks, Kubeflow, Amazon SageMaker, etc.
This includes respecting the privacy of the data holder and ensuring its data security. It also includes maintaining a secure way of transferring data to avoid any hindrance and maintaining transparency in methodologies and results. By following these principles, data scientists can ensure that their work benefits society without causing harm or infringing on individuals’ rights.
Mention your approach. Proper documentation is a must when we talk about transparency. It also involves clarifying model choices and forecasts and granting access to model code and documentation. Prioritising transparency allows data scientists to establish trust with stakeholders and guarantee accountability in their work.
Gaining consent for data collection is an important part of ethical data collection. Furthermore, gathering only the required data, with no additional data collection that can affect the privacy of anyone, is a good practice. Adhering to data protection laws, transparent data transfer, and respectful and mutual sharing of data are some good practices.
Regularly checking all the following key practices helps in maintaining the quality of your data:
In conclusion, these interview questions have a wide range of applications all over the data science field. Both the technical and nontechnical aspects of interview questions are covered. Interviewers would easily generate new questions. Hence, it all depends on how well you prepare. Hope these questions helped you to get an idea of what the interviewer asks.
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