Confidence Interval Calculator (Snapshot)

What is this?

This tool calculates a confidence interval (CI) for the difference in conversion rates between two groups (typically a control and a variant) using just the final totals for each.

Why use it?

P-values only tell you whether a result is statistically significant. Confidence intervals tell you how big the difference might be, and how confident you can be about that range.

Use this tool to:

• Quantify the uncertainty around your observed uplift
• See if the interval includes zero (no effect)
• Determine whether the possible outcomes are meaningfully positive or negative

How it works

You enter:

• Total conversions and visitors for the control group
• Total conversions and visitors for the variant group
• Desired confidence level (e.g. 95%)

The calculator:

• Computes conversion rates
• Calculates the absolute and relative lift
• Generates a confidence interval (CI) around the difference using the Wald method

Formula used

The CI is calculated on the difference of proportions:

Where:

• $\hat{p}_1$ and $\hat{p}_2$ are the observed conversion rates
• $SE$ is the standard error of the difference in proportions
• $Z_{\alpha/2}$ is the z-score corresponding to your confidence level (e.g. 1.96 for 95%)

If the CI includes 0, the result is not statistically significant at the selected confidence level.

Use this to get a range of plausible outcomes, not just a binary yes/no.