Type I Error

Type I Error is a false positive result that occurs when an A/B test incorrectly concludes there is a significant difference between variations when no true difference exists.

Meaning & Context

Also known as a false positive or alpha error, this statistical mistake happens when you reject the null hypothesis even though it's actually true. In A/B testing, this means declaring a winner and implementing changes based on what appears to be a significant result, when the observed difference was actually due to random chance. The probability of making a Type I Error is controlled by your significance level (alpha).

Why It Matters

Type I Errors can lead to costly business decisions based on false insights, causing you to invest resources in implementing changes that won't actually improve conversion rates. Understanding and controlling for Type I Errors helps maintain the integrity of your testing program and prevents you from drawing incorrect conclusions that could harm performance. Most A/B testing platforms set alpha at 0.05, meaning you accept a 5% risk of false positives.

Example

Your A/B test shows that a new checkout button increased conversions by 15% with p < 0.05, so you implement it site-wide. However, the lift was actually due to random variation, and after implementation, you see no sustained improvement in actual conversion rates.