S scores are unbiased and quantitative. (a) Distribution of S scores for pairs of genes whose individual mutations give different growth phenotypes. The curves represent scores from pairs of genes whose individual mutations both yield slow growth phenotypes (blue circles), both yield growth phenotypes typical of our set of mutant strains (green triangles), and both yield relatively fast growth phenotypes (red squares). (b) Median interaction score on the second measurement (from an independent construction of strains) for pairs of genes with the indicated score on the first measurement. (c) Histograms of the observed colony size divided by the expected colony size for double mutant strains with S scores of approximately -3 (blue), -5 (green), -10 (red), and -20 (brown).