Glossary: Statistical Inference and A/B Testing

Agresti-Coull interval

Also written: plus-four interval, add two and two

A confidence interval for a proportion that stays sensible with small counts by adding about two yeses and two noes to the data before calculating. The simpler 'plus four' version (add 2 yeses and 2 noes, then use the usual formula) turns Alex's 2 broken mugs in 40 into an interval of 0.6% to 17.6% instead of a impossible negative lower bound.