Interview questions for data scientists

Data science is one of the most ill-defined fields in tech. May this article 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 early.

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?

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. 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 / optimization / numerical methods…

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

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 foster the discussion: there is seldom a unique right answer!

Machine learning and model engineering

  • Which algorithms exist for the task you are currently discussing?
  • When are they good, good enough, good benchmarks, etc.
  • How do they work? How do they scale? How is their parameter fitting optimized?
  • How do you 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 do you do pick the final model ? Performance? training-time? evaluation speed? complexity?
  • Understanding cross-validation and bias/variance is critical.
  • How do you do variable and model selection? You can talk about information-based metrics, regularization, sparsity-inducing methods like L1 regularization, forward/backward search…

Data engineering

  • How would you design X?
  • What preprocessing would you do for task X?
  • What about unbalanced data? missing data? outliers?
  • And categorical variables of high cardinality…?
  • How does one deal with big data? Talk about online learning, map/reduce, (mini-batch) stochastic gradient descent…
  • What tools/languages do you know to achieve X? Which ones did you use and why?
  • Which visualization tools do you use? Why?

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.

Arthur Flam

Entrepreneur, data scientist. Now experimenting new projects at ECI Telecom's CTO office.

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