Shepperd and MacDonell "Evaluating prediction systems in software project estimation". Information and Software Technology 54 (8), 820-827, 2012, proposed an improved measure of the effectiveness of predictors based on comparing them with random guessing. They suggest estimating the performance of random guessing using a Monte?Carlo scheme which unfortunately excludes some correct guesses. This biases their MAR P 0 to be slightly too big, which in turn causes their standardised accuracy measure SA to over estimate slightly. In commonly used software engineering datasets it is practical to calculate an unbiased MAR P 0 exactly.