A z-score (aka “standard score”) is a number that compares the relationship of a particular value to the average in a group of values. On a normal distribution curve, that relationship is described using “standard deviation,” meaning distance from the mean value. A z-score is used to figure out whether the data you’ve collected in an A/B test, for example, can be attributed to chance, or if it’s actually the result of the changes you’ve made.
