Explain difference-in-differences with an example
Last updated: December 28, 2025
Quick Overview
Explain difference-in-differences in simple terms and provide a concrete example.
Shopify
December 28, 20255
5
1,791 solved
Explain difference-in-differences in simple terms and provide a concrete example.
This statistics question from Shopify's Phone Screen tests your ability to apply mathematical reasoning to practical problems. The interviewer expects precise definitions, correct methodology, and awareness of assumptions and limitations.
What the Interviewer Expects
- Set up the problem formally with proper notation
- Apply the correct statistical test with clear justification
- Interpret results with appropriate caveats and confidence levels
- Discuss practical significance vs statistical significance
- Identify potential confounders and how to address them
Key Topics to Cover
How to Approach This
- Define your hypotheses (H0 and H1) clearly before performing any test.
- Calculate required sample size BEFORE running an experiment, using power analysis.
- Remember the Central Limit Theorem: sample means become approximately normal with large n.
- Watch for Simpson's paradox. Always segment data by key dimensions.
- Distinguish between statistical significance and practical significance.
Possible Follow-up Questions
- What alternative statistical method could you use here?
- What if the sample size is very small?
- How would you design a follow-up experiment based on these results?
- How would you handle multiple comparisons?
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