Correction to: Nature Methods, published online 30 March 2017

The published version of this article contains a mathematical error. In the discussion of z tests in the fourth paragraph, the same z score is incorrectly given for both AA and aa; the last two sentences of the paragraph should read as follows: “For example, for AA the z score of the difference is 3.75/√6.25 = 1.5, yielding P = 0.13. However, for aa the same computation yields –16.25/√56.25 = –2.167 and P = 0.03.”