Details on how to compute the P68 percentile Harvey Motulsky, GraphPad Software 5 December 2006 Our method depends on computing the 68.72 percentile of the absolute value of the residuals, which we call P68. We neglected to document how we computed this percentile. Hyndman and Fan (The American Statistician, 50: 361-365, 1996) document nine different methods for computing percentiles. We use the method termed 'P7' in that paper, which is the same method used by Excel 2003. Competing interests I am an author of the paper.