If you're planning to run an experiment – either on the web or in real-life – it's critical to collect the right amount data so that you know the experiment results are scientifically valid.

A/B Buddy is a sample-size calculator that helps you design A/B experiments with a quick slide of a finger. It uses a power-analysis formula to tell you exactly how many subjects, or users, that you'll need in an experiment in order to detect a statistically significant difference between two groups. Because if you have too few users in the experiment, you'll never see any difference between "A" and "B"; if you have too many enrolled, the experiment will never end. A/B Buddy tells you exactly the right number.

How does it work? Let's say your current conversion rate is 10%, and you want to see if a proposed change from the design team can boost that number up to 12% – that is, a 20% lift in the conversion rate. Simply pop open A/B Buddy, set the Baseline conversion rate to 10%, and the Minimum Detectable Effect to 20%, and your answer is waiting for you at the top of the screen.

A/B Buddy includes a Share button, which links to a web page displaying your sample size calculation. Your numbers, however, remain 100% private – all the information is stored in the URL fragment, and is never transmitted to any servers.

By default, A/B Buddy uses the most common statistical thresholds: 5% for the significance level (false-positive rate), and 80% for the power level (ability to detect the Minimum Detectable Effect). Power users can adjust these values after unlocking the Pro Features in-app purchase. But most users will find the defaults adequate for their needs.

Questions? Suggestions? Ideas? Feedback is always welcome at

What’s New

Version 1.0.1

• NEW glossary of terms explaining what each number represents
• NEW allow Minimum Detectable Effects of up to 100% (previous limit was 50%)
• Fix a bug when copying the subject count

Ratings and Reviews

3.0 out of 5
2 Ratings

2 Ratings


Evan Miller
8.9 MB

Requires iOS 10.0 or later. Compatible with iPhone 5s, iPhone 6, iPhone 6 Plus, iPhone 6S, iPhone 6S Plus, iPhone SE, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, iPhone X, iPhone XS, iPhone XS Max, iPhone XR, iPad Air, iPad Air Wi-Fi + Cellular, iPad mini 2, iPad mini 2 Wi-Fi + Cellular, iPad Air 2, iPad Air 2 Wi-Fi + Cellular, iPad mini 3, iPad mini 3 Wi-Fi + Cellular, iPad mini 4, iPad mini 4 Wi-Fi + Cellular, 12.9-inch iPad Pro, 12.9-inch iPad Pro Wi-Fi + Cellular, iPad Pro (9.7‑inch), iPad Pro (9.7-inch) Wi‑Fi + Cellular, iPad (5th generation), iPad (5th generation) Wi‑Fi + Cellular, iPad Pro (12.9‑inch) (2nd generation), iPad Pro (12.9‑inch) (2nd generation) Wi‑Fi + Cellular, iPad Pro (10.5‑inch), iPad Pro (10.5-inch) Wi‑Fi + Cellular, iPad (6th generation), iPad Wi-Fi + Cellular (6th generation), iPad Pro (11-inch), iPad Pro (11-inch) Wi-Fi + Cellular, iPad Pro (12.9-inch), iPad Pro (12.9-inch) Wi‑Fi + Cellular, iPad mini (5th generation), iPad mini (5th generation) Wi-Fi + Cellular, iPad Air (3rd generation), iPad Air (3rd generation) Wi-Fi + Cellular,and iPod touch.



Age Rating
Rated 4+
© 2019 Evan Miller
In-App Purchases
  1. Pro Features $8.99


  • Family Sharing

    With Family Sharing set up, up to six family members can use this app.

More By This Developer

You May Also Like