Histograms of p values. (a) Distribution of p values for chromosome 21 genes (253 probe sets represented on the microarray). The histogram contains 20 bins, at intervals of 0.05. The expected number of genes in each bin by chance alone is 253 × 0.05 = 12.65 (horizontal line). (b) Distribution of p values for non-chromosome 21 genes (20,008 probe sets). The expected number of genes having a p value < 0.05 by random chance is 20,008 × 0.05 = 1000.4 (horizontal line).