Explain confidence interval with an example
Last updated: August 21, 2025
Quick Overview
Explain confidence interval in simple terms and provide a concrete example.
Supabase
August 21, 202515
5
2,532 solved
Explain confidence interval in simple terms and provide a concrete example.
Supabase 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
- How would you handle an experiment where the control and treatment groups are different sizes?
- How would you handle interference between treatment and control?
- How would you handle seasonality in your experiment?
- What would you do if a stakeholder wants to end the experiment early because initial results look good?
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Problem Setup
To explain confidence intervals, we need to first define what we mean by a confidence interval. In the context of Supabase, let's consider an experiment where we want to measure the effect of a new fe...
Methodology
To calculate the confidence interval for the difference in average sign-ups between the control and treatment groups, we will use the following steps:
- Calculate the mean sign-ups for both groups. 2...
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