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Microsoft·Data Scientist·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Got a machine learning theory question at Microsoft, just the one. Short and focused, felt more like a screen than a deep technical dive.

Questions Asked (1)

Q1

Is Mean Squared Error (MSE) an appropriate cost function to use when training a logistic regression model?

Technical Trade-offsProduct Analytics & Metrics
Author's notes

Said no pretty quickly, which was right, but then stumbled trying to explain why in a clean way.

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AI HintsAI Generated

Suggested Approach

Start by clarifying that MSE is not typically appropriate for logistic regression because it leads to non-convex optimization and poor probabilistic calibration. Explain the theoretical and practical reasons, then contrast with the standard cross-entropy loss, highlighting why it is preferred.

Pro tip: Mention that while MSE can technically be used with logistic regression, it often results in slow convergence and suboptimal probability estimates, which is why cross-entropy is the industry standard.

1. Define the context

Briefly explain what MSE and logistic regression are, and why the choice of cost function matters for model training.

2. Discuss theoretical issues

Explain that MSE with logistic regression leads to a non-convex cost surface, making optimization prone to local minima and slow convergence.

3. Highlight practical implications

Mention that MSE does not align with the probabilistic nature of logistic regression, often resulting in poorly calibrated probabilities and reduced classification performance.

4. Introduce the standard alternative

Describe cross-entropy (log loss) as the appropriate cost function, emphasizing its convexity and maximum likelihood foundation.

5. Conclude with a recommendation

Summarize that MSE is generally not recommended for logistic regression, and cross-entropy should be used for better optimization and probabilistic outputs.

Key Points to Mention

  • Non-convexity of MSE with logistic regression
  • Cross-entropy as the maximum likelihood estimator
  • Gradient descent convergence issues with MSE
  • Probabilistic calibration and interpretation
  • Common industry practice and standard implementations
  • Potential use of MSE in rare cases (e.g., regression-like tasks) but with caveats

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.