Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About EY hiring for Data Science roles
EY is one of the Big Four professional services networks, offering assurance, tax, strategy and transactions, and consulting services. In India, EY GDS and EY member firms support global clients with technology consulting, data analytics, cybersecurity and risk work. It hires for analyst and consultant roles in data analytics, BI, testing, ERP and risk.
For Data Science roles, this EY practice set focuses most on Machine Learning (7), Statistics and Probability (7), Python for Data Science (6), MLOps and Model Deployment (5). It has 7 easy, 31 medium, 2 hard questions, so you can start easy and work up to the harder ones.
Company type
Consulting
Headquarters
London, United Kingdom
Main India offices
Bengaluru, Kolkata, Gurugram, Hyderabad, Kochi
EY Data Science interview process
Online assessment (aptitude + technical)
Technical interview
Managerial or case round
Partner or HR round
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
Solved for EYStart practicing
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IoU measures how well a predicted box or mask overlaps the ground truth. It is the area of their intersection divided by the area of their union, giving a value from 0 for no overlap to 1 for a perfect match. In detection, a prediction usually counts as correct if IoU is at least 0.5 with a ground truth box of the same class. IoU is also used inside non-maximum suppression.
It predicts from the k nearest labelled points. It is simple but slow at prediction time, sensitive to scaling and irrelevant features, and weak in high dimensions.
03
Explain exponential smoothing and the Holt-Winters method.
Simple exponential smoothing forecasts with a weighted average of past values where weights decay exponentially, so recent points matter more. Holt's method adds a smoothed trend component. Holt-Winters adds a seasonal component as well, either additive or multiplicative. Each component has a smoothing parameter between 0 and 1 that is fitted to the data. These models are fast, easy to explain and strong baselines for retail and demand data.
04
After one-hot encoding you have 2,000 features and training is slow. What do you do?
Find which columns caused the explosion, usually a few high-cardinality categoricals. Group rare categories into Other, or switch those columns to frequency, target or hashing encoding. Use sparse matrices so zeros take no memory. Tree libraries like LightGBM and CatBoost can handle categoricals natively without one-hot. Then remove near-constant columns and check that validation scores hold up with the smaller feature set.
Regularization adds a penalty on weights to reduce overfitting. L1 (Lasso) adds |w| and can push features to exactly zero. L2 (Ridge) adds w² and shrinks weights smoothly.
06
Your model performance degrades every few months. Would you retrain on a fixed schedule or based on triggers?
Scheduled retraining, like monthly, is simple and predictable but may be too late after a sudden change or wasteful when nothing changed. Trigger-based retraining runs when monitoring shows drift or a performance drop, which reacts faster but needs reliable monitoring. Many teams combine both: a regular schedule plus triggers for big shifts. Either way, a retrained model must pass the same evaluation gate before replacing the current one.
07
What are embeddings and vector databases, and why do LLM apps use them?
An embedding model turns text into a vector so that similar meanings are close in space. A vector database stores these vectors with metadata and finds the nearest ones to a query vector quickly using approximate nearest neighbour indexes like HNSW. LLM apps use them for semantic search and retrieval-augmented generation, finding the most relevant documents to put into the prompt. Examples include pgvector, FAISS and dedicated vector stores.
08
What is effect size and why should you report it along with a p-value?
Effect size measures how large a difference is, for example the absolute lift in conversion or Cohen's d for means. A p-value only says whether the difference is likely due to chance. With millions of users, a 0.01 percent lift can be significant yet useless for the business. Reporting effect size with a confidence interval tells decision makers both whether the effect is real and whether it is worth acting on.
09
You must classify support messages into 40 categories, but some categories have only 20 examples. What do you do?
Start with a pretrained sentence embedding model plus a simple classifier, or fine-tune a small transformer with class weights. For the rare categories, try few-shot approaches such as SetFit or an LLM with examples in the prompt, and generate or collect more examples. Merge categories that are rarely distinguishable. Report per-class F1 rather than overall accuracy, and route low-confidence predictions to a human queue.
10
How do you work with domain experts to find good features?
Situation: on an insurance claims model, my first features gave average results. Task: improve fraud detection. Action: I sat with two claim investigators for a day and asked what made them suspicious. They mentioned claims filed soon after policy start and repeated garage names. I turned these into features and checked them with the team. Result: recall at the same precision rose by 12 points. Experts often know signals the data hides.
11
Your A/B test shows a significant lift after 2 days. The product team wants to ship now. What do you say?
I would ask them to wait for the planned duration. Checking early and stopping on the first significant result, called peeking, inflates false positives. Two days also misses weekly patterns and novelty effects, where users click a new feature just because it is new. If speed matters, we can use a sequential testing method designed for early stopping. Otherwise we run the full sample size calculated before launch.
A CTE, common table expression, is a named temporary result defined with WITH and used in the query that follows. It breaks complex SQL into readable steps and can be referenced more than once. A recursive CTE refers to itself: an anchor query starts the result and a recursive part keeps adding rows until none are added. It is used for hierarchies like employee-manager chains or generating date series.
15
Training a transformer on a GPU fails with an out-of-memory error. What can you do?
Reduce the batch size and use gradient accumulation to keep the same effective batch. Shorten the maximum sequence length. Turn on mixed precision training with float16 or bfloat16. Use gradient checkpointing, which saves memory by recomputing activations. Freeze some layers or switch to a smaller model, or use parameter-efficient fine-tuning like LoRA. Also make sure no old tensors stay on the GPU between runs.
ANOVA, analysis of variance, tests whether three or more group means are equal. It compares variation between groups to variation within groups using an F statistic. For example, comparing average order value across four city tiers. A significant result says at least one mean differs but not which one, so you follow up with post-hoc tests like Tukey's HSD. It assumes independent samples, roughly normal residuals and similar variances.
17
What is statistical power and how can you increase it?
Power is the probability of detecting a real effect. Increase it with a larger sample, a bigger effect size, lower noise or a higher significance level.
The ROC curve plots true positive rate against false positive rate across thresholds. AUC is the probability that a random positive is ranked above a random negative; 0.5 is chance.
19
Pandas: find the top 3 products by revenue in each region.
The kernel is a small learnable filter, such as 3x3, that slides over the image and computes a weighted sum at each position. Stride is how many pixels it moves each step; stride 2 halves the output size. Padding adds border pixels so the output can keep the same size and edge pixels get processed. Output size equals (input minus kernel plus 2 times padding) divided by stride, plus one.
22
A 10 million row DataFrame with a city column is using too much memory. How can the category dtype help?
The category dtype stores each unique value once and keeps small integer codes for every row. For a column like city with 50 unique values in 10 million rows, this can cut memory by more than 90 percent and speed up groupby and sorting. It helps when a column has low cardinality. It does not help for unique IDs or free text, and adding new categories later needs extra handling.
23
Tell me about a time you found a mistake in your own analysis after sharing it.
Situation: after presenting a retention report, I noticed I had counted test accounts as users. Task: correct it before decisions were made. Action: I reran the numbers that evening, informed my manager and the audience with a short note showing old and new figures and what changed, and added a filter and a check to the shared query. Result: the conclusion held with smaller numbers, and the team valued the transparency.
24
What is correlation and why does it not imply causation?
Correlation measures linear association between variables (-1 to 1). A third factor, reverse causality or chance can create it, so causation needs controlled experiments.
25
Tell me about a time you stopped a team from shipping a change based on a flawed experiment.
Situation: a team wanted to launch a feature after a test showed a 6 percent lift. Task: as the reviewing data scientist, validate it. Action: I found that assignment was triggered after users clicked into the feature, so treatment had only engaged users. I explained the selection bias simply, proposed assigning at app open, and helped rerun it. Result: the true lift was under 1 percent, and we added assignment checks to the review template.
All three are gradient boosting libraries. XGBoost grows trees level by level with strong regularisation and is very mature. LightGBM uses histogram binning and leaf-wise growth, so it is usually fastest on large data but can overfit small data without limits on leaves. CatBoost handles categorical features natively with ordered target statistics and uses symmetric trees, so it gives good results with little tuning. Choice depends on data size, categoricals and latency needs.
27
What is the difference between list, tuple, set and dict in Python?
List: ordered and mutable. Tuple: ordered and immutable (hashable, slightly faster). Set: unordered, unique items, fast membership checks. Dict: key–value pairs with fast lookup by key.
28
Your deployed model gives different predictions than the same model in your notebook. Why?
Usually preprocessing differs between training and serving. Check that the same pipeline object was saved, not just the estimator, and that category order, missing value handling and scaling match. Compare library versions, feature types like strings versus integers, column order, and time zones in date features. Send the same few rows through both paths and compare features step by step to find the first mismatch.
29
You keep adding features and R-squared keeps rising. Is the model really getting better?
R-squared shows the share of variance in the target explained by the model. It never decreases when you add a feature, even a useless one. Adjusted R-squared penalises extra features based on the number of predictors and sample size, so it rises only if a new feature improves the model more than chance would. Use adjusted R-squared to compare linear models with different numbers of features.
30
What are dataclasses in Python and why are they useful in data projects?
A dataclass, created with the @dataclass decorator, automatically generates __init__, __repr__ and __eq__ from annotated fields. It is a clean way to hold structured records such as a training config with learning rate, batch size and paths. Options like frozen=True make instances immutable, and default values reduce boilerplate. Compared with plain dictionaries, dataclasses give attribute access, type hints and clearer errors when a field is misspelled.
31
Write Python code to compare revenue per user between two groups using Welch's t-test.
With so few defects, a plain classifier will learn to say no defect. Collect every defect image possible and oversample them or use class-weighted or focal loss. Consider anomaly detection trained only on good parts, which flags anything unusual. Use augmentation that mimics real variation in lighting and position. Evaluate with recall at a fixed false alarm rate, since missing a defect usually costs far more than a manual recheck.
It tries candidate splits and picks the one that most reduces impurity — Gini or entropy (information gain) for classification, variance / MSE for regression.
MLOps is the set of practices that take machine learning models from experiments to reliable production and keep them working. It covers versioning data and code, tracking experiments, automated training pipelines, testing, deployment, monitoring and retraining. It borrows ideas from DevOps but adds data and model concerns, because a model can fail even when the code has not changed, simply because the data has.
35
You target-encoded a city column and validation AUC jumped from 0.72 to 0.95. What do you suspect?
Target encoding replaces each category with the average target value for that category, for example the churn rate per city. It handles high-cardinality columns compactly. The main risk is leakage: if a row's own target is included in its encoding, the model sees the answer and overfits. Prevent it with out-of-fold encoding, smoothing toward the global mean for rare categories, and fitting only on training data.
36
Your dataset is highly imbalanced (2% positives). How do you handle it?
Use stratified splits, class weights, resampling (SMOTE or undersampling) and threshold tuning, and judge the model with PR-AUC or F1 instead of accuracy.
They store same-type values in contiguous memory and run vectorised operations in compiled C, avoiding Python loop overhead and per-object type checks.
38
Write Python code to compute a 95% bootstrap confidence interval for the mean.
Situation: our document classifier mislabelled several legal notices, and the client was upset. Task: explain clearly and restore confidence. Action: I showed the misclassified examples, explained that they used a format absent from training data, and avoided jargon. I proposed adding those samples, a confidence threshold for human review and weekly error reports. Result: errors on that format fell sharply and the client extended the project.
40
What is the difference between saving a model with pickle, joblib and ONNX?
pickle serialises Python objects; it is simple but tied to Python and library versions, and loading untrusted pickle files is a security risk. joblib is similar but more efficient for objects with large NumPy arrays, like scikit-learn models. ONNX is an open, language-neutral format that runs in optimised runtimes across languages and hardware, useful for fast serving. Always store library versions alongside pickle or joblib files.
Tips to crack the EY Data Science interview
Practise explaining a data project in business terms, focusing on the problem, approach and recommendation for the client.
Revise Excel, SQL and Power BI, which appear frequently in EY analytics and consulting job descriptions.
Read about EY's service lines and its focus on technology, AI and sustainability consulting for fit questions.
Keep a one-minute story ready for every project on your CV: problem, your part, tools, result.
Practise the hard questions on this page aloud; EY interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the EY Data Science interview?
Candidates usually go through 4 stages: Online assessment (aptitude + technical); Technical interview; Managerial or case round; Partner or HR round. The exact rounds change with the role, team, location and your experience, so confirm the process with the recruiter.
Which topics should I prepare for the EY Data Science interview?
This practice set leans on Machine Learning (7), Statistics and Probability (7), Python for Data Science (6), MLOps and Model Deployment (5). Revise those first, then practise explaining one or two of your own projects end to end, because most EY interviewers ask you to walk through real work.
Are these the exact questions asked at EY?
No. These are curated practice questions that match the skills and difficulty usually tested in consulting interviews like EY's. They are not a leaked or verified list of past questions.
Where does EY hire Data Science professionals in India?
EY has large teams in Bengaluru, Kolkata, Gurugram, Hyderabad, Kochi. Openings and locations change often, so check the EY careers page and filter by city and experience.