Calculate probability for A/B test results
Last updated: May 2, 2026
Quick Overview
Given the following scenario about revenue per session, calculate the the sample size needed.
Vercel
May 2, 20265
4
658 solved
Given the following scenario about revenue per session, calculate the the sample size needed.
Vercel asks this during the Phone Screen to assess your experimentation skills. They want to see how you define success metrics, design controlled experiments, and interpret results with appropriate statistical rigor.
What the Interviewer Expects
- Design a rigorous experiment with proper randomization and sample size calculation
- Define primary and guardrail metrics with clear rationale
- Address novelty effects, network effects, and interference
- Segment results appropriately and identify heterogeneous treatment effects
- Propose follow-up analyses when results are ambiguous
Key Topics to Cover
How to Approach This
- Define success metrics carefully. A good metric is measurable, actionable, and aligned with business goals.
- Run experiments long enough to account for novelty effects and weekly seasonality.
- Use funnel analysis to identify where users drop off for maximum optimization impact.
- Segment results by key dimensions (platform, country, user cohort) to catch hidden patterns.
- Consider network effects and interference between treatment and control groups.
Possible Follow-up Questions
- What if you discover a bug in the logging during the experiment?
- How would you handle an experiment where the control and treatment groups are different sizes?
- What would you do if a stakeholder wants to end the experiment early because initial results look good?
- How would you handle seasonality in your experiment?
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Browse Analytics QuestionsSample Answer
Problem Setup
The analytical question is: What sample size is required to detect a statistically significant difference in revenue per session between the control and treatment groups in an A/B test? To do this, we...
Methodology
To calculate the sample size needed for the A/B test, we can use the formula for sample size calculation for comparing two means:
[ n = \frac{(Z_{\alpha/2} + Z_{\beta})^2 \cdot (\sigma_1^2 + \sigma_...
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