Interview practice

Data Scientist / ML Engineer

2–5 years · Mid — Python · Statistics · scikit-learn · Feature engineering · Model evaluation · Deployment

0–2 years 2–5 years 5–8 years 8+ years
technical Debugging 1 of 6
During testing, you find that your model predictions are consistently biased towards the lower end of the target variable. How do you approach diagnosing this bias?
What a strong answer covers

Examines the training data for skewness or outlier effects; checks for data leakage or improper target encoding; evaluates the model's assumptions and performance metrics on different segments; considers alternative algorithms if necessary.

The follow-up you would get

What role does the choice of loss function play in this bias?

Could you have answered that?
technical Data access patterns 2 of 6
You are working with a large dataset that requires frequent access during model training. What strategies do you use to optimise data access patterns?
What a strong answer covers

Utilises data partitioning or sharding to improve access speeds; leverages in-memory data structures or databases; implements efficient data loading techniques such as lazy loading; evaluates batch processing to reduce overhead.

The follow-up you would get

How do you handle feature engineering in a streaming data scenario?

Could you have answered that?
technical Deployment 3 of 6
After deploying a machine learning model to production, the API response time increases significantly. How do you investigate the cause and what optimisations do you consider?
What a strong answer covers

Monitors the API logs to identify response bottlenecks; checks for model loading time and input data preprocessing efficiency; investigates the hardware and resource utilisation on the server; considers caching predictions for repeated requests.

The follow-up you would get

If the model is too large to cache, what alternative solutions do you explore?

Could you have answered that?
scenario Model Monitoring 4 of 6
Your model has been live for a month, and you notice a gradual decline in performance metrics. Walk me through your investigation process.
What a strong answer covers

Review model performance metrics over time to identify trends; check for concept drift using statistical tests; compare recent input data distributions to training data; evaluate external factors influencing predictions; determine if retraining is necessary and plan for data collection.

The follow-up you would get

You discover significant changes in user behaviour affecting the model's primary features. How do you adjust your strategy to mitigate this?

Could you have answered that?
technical Model evaluation 5 of 6
You deploy a classification model, and after a week, you notice a significant drop in accuracy from 90% to 60%. What steps do you take to diagnose and rectify the issue?
What a strong answer covers

Checks the data for concept drift; compares the incoming data distribution to the training data; evaluates model predictions against new labels if available; considers retraining the model with updated data or features.

The follow-up you would get

What tools or methods do you use to detect concept drift?

Could you have answered that?
ownership Model evaluation 6 of 6
Tell me about a time you had to choose a metric for evaluating your model's performance. What considerations influenced your choice?
What a strong answer covers

Specifies the project context and model type; identifies the chosen metric and justifies its relevance to the business problem; discusses alternative metrics considered and why they were not selected; reflects on any surprises during evaluation post-deployment; avoids vague references to team consensus.

The follow-up you would get

How did you validate the effectiveness of your chosen metric?

Could you have answered that?
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