/ #datascience #recruiting

Interview questions for data scientists

Paul Cézanne

Data science is one of the most ill-defined fields in tech. This article may inspire you for your next interview, as a recruiter or as a candidate!

Recruiters have work to do

Interviews are hard, and all the more so in data science. Every company has a different opinion on what data science is. Worse, many figure out what they want during/after the hiring process.

Your responsibility as a recruiter is to give a job description as clear as possible: do you need a data engineer, a visualization expert, a data analyst, an algorithm engineer, or a machine learning researcher? Know what you want, and filter accordingly.

This article deals with “machine learners” data scientists.

Uncertainties about the job will make the best candidates flee – unless you’re Google/etc. They’ll want to understand what tooling is built already, how their work’s ROI will be measured, who is in the team…

Make data scientists talk about what they know

Data scientists have very diverse backgrounds. It is impossible to prepare for each interview: should they study “standard” computer science? statistics? bayesian things? deep learning? machine learning? your company’s field?

Ask a data scientists why they think they are one; you’ll get many different answers, it’s good!

The good candidates have a basic knowledge of many topics, are hands-on, and have strong knowledge in some domains. Talk about those if you want to have technical discussions. Ask them what is their secret sauce! Some examples:

  • Problem types: regression / classification / clustering / anomaly detection…
  • Data types: computer vision / time series / NLP / recommendation…
  • Complex data: dimensionality-reduction, manifold learning…
  • Points of view: deep learning / bayesian machine learning / graphical models…
  • Expertise: industry insights / visualization / optimization / numerical methods…

Candidate with industry experience should have failed projects under their belt. Those lessons are essentials 😄

Maybe it is best to avoid data science tests

Many companies have candidates discuss logistic regressions and basic bayesian concepts for an hour. Personally I feel those tests only check if you reviewed your ML textbook in the last 6 months… If you need ML/statistics engineers, filter more on the CV and use to interview time to check more important things.

Data science is about practice, engineering, judgment when selecting approaches and tools.

My opinion is that “open” 1-1 interviews are better than written in-office tests. Many companies give take-home datasets and ask for an analysis. It can help show who is down-to-earth! For experienced candidates it can be a lot of work to ask - ask instead about their projects, or presence on Kaggle/Github…

Topics for data science interviews

Still, many topics seem fair game in data science interviews. Remember that the goal is to get the conversation started: there is seldom a unique right answer!


Working in a structured manner is all the more important that data scientists work with other teams. Their work will need to be understandable and reproducible. You may want to know:

  • How they handle the coding process from prototyping to production?
  • How they improve upon your models and debug them?
  • What tools they use?
  • How they document their results, projects and infrastucture?
  • How they work with other teams (product/sales/qa/frontend/backend/devops/hardware…)?

Senior data scientists and managers should be able to lead those issues.

Machine learning and model engineering

Feature engineering is a central piece of data science projects. It is worth talking about:

  • How to discover good features and preprocessing.
  • How to deal with unbalanced data, missing data, outliers or even categorical variables of high cardinality…

Model selection is also a big topic, with much to investigate:

  • Which algorithms exist for either a project they worked on, or one relevant for you?
    • How to decide which to try?
    • When are models good, good enough, good benchmarks, etc?
    • How does those models work? How do they scale? How are their parameters optimized?
    • How can we do hyperparameter tuning?
  • How can we rate a model’s performance ?
    • Depending on the context, this can lead to discussions about false positives/negatives, accuracy, recall, AUC, lift… How should one decide?
    • How to pick the final model ? Performance? training-time? evaluation speed? complexity?
    • Understanding cross-validation and bias/variance is critical. Candidates should be able to say a few words. With deep learning, things get more difficult to understand though…
  • How should we do variable and model selection? You about information-based metrics, regularization, sparsity-inducing methods like L1 regularization, forward/backward search…
  • Is it just alchemy and black magic? Can we just cross-validate everything and stop there?

Data engineering should not be the focus of the discussion, but it is interesting to know if candidates have not only wrote scripts but also built tools or systems. Depending on your use-cases, you could expect exposure to big data friendly techniques (online learning, map/reduce, stochastic gradient descent, dark corners of linear algebra…) or tools (often spark, hadoop…).

Making a decision?

It is a always a leap of faith. How can you reliably assess candidates' involvement in past projects, their breadth of expertise, or even how well you’d work with them, all in under a few hours?

Don’t be afraid to pick candidates with different backgrounds and levels of experience. Data science teams need this. If you are starting such a team, rely on someone with expertise: it is the only way to avoid wasting time with over-hyped tools.