The I² statistic is a test of heterogeneity. I² can be calculated from Cochrane Q (the most commonly used heterogeneity statistic) according to the formula: I² = 100% × (Cochrane Q – degrees of freedom) / Cochran Q. Any negative values of I² are considered equal to 0, so that the range of I² values is between 0% and 100%.

A common test for heterogeneity that assumes the null hypothesis that all the apparent variability between individual study results is due to chance. Cochrane Q generates a probability, presented as a P value, based on a ² distribution, that between-study differences in results equal to or greater than those observed are likely to occur simply by chance.