Cross-hybridization distorts P values for null genes in the Latin Square dataset. Shown are the results of the Kolmogorov-Smirnov test for the null genes for all 91 Latin Square comparisons as a function of the average difference in spike concentration (see text). The null hypothesis for the Kolmogorov-Smirnov test is that the observed P values are identical to a uniform distribution. Error bars are standard errors. The red line is the P = 0.05 level.