In using the NPD ratio as a surrogate for the coefficient of coincidence, the author (yours truly) is running a risk of unknown magnitude. Unlike the coefficient of coincidence, the NPD ratio is dependent on the assumption that the first crossover in the interval does not significantly reduce the number of sites available for a crossover between the other two chromatids. The concentration of DSB’s at hotspots in yeast may be a problem, depending on the extent to which crossing over between two of the chromatids reduces the rate of DSBs at the same hotspot on the other two chromatids or disallows the use of such DSBs for crossing over by either pathway. In so far as the author’s assumption is unjustified, his NPD-based calculations underestimate the fraction of noninterfering crossovers.