Explain confidence interval with an example

Last updated: August 23, 2025

Quick Overview

Explain confidence interval in simple terms and provide a concrete example.

Stripe

Analytics & Experimentation

Data Scientist

Stripe

August 23, 2025

Data Scientist

Phone Screen

Analytics & Experimentation

Medium

4,821 solved

Explain confidence interval in simple terms and provide a concrete example.

This analytics question from Stripe's Phone Screen tests your ability to think critically about data. The interviewer expects you to consider confounding variables, selection bias, and the difference between correlation and causation.

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

Novelty and primacy effects

Sample size and power calculation

Simpson's paradox and ecological fallacy

Metric definition and success criteria

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 interference between treatment and control?
What would you do if a stakeholder wants to end the experiment early because initial results look good?
What if you discover a bug in the logging during the experiment?

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Sample Answer

Problem Setup

To explain confidence intervals, we need to analyze a specific metric, such as the average transaction value processed by Stripe. Our analytical question could be: "What is the 95% confidence interval...

Methodology

To calculate the confidence interval for the average transaction value, we will use the formula for a confidence interval:

$CI = \bar{x} \pm z \times \frac{s}{\sqrt{n}}$

Where:

( \bar{x} ...

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