Density distribution histograms. (a) Example of bimodal density distribution of log2 intensities and histogram of merged log2 intensities. The merged level with fewest segments assigned to it is chosen as the cutoff value. All segments with merged values above this cutoff are predicted as present. An arrow indicates the cutoff level for this particular sample. (b) Example of unimodal (or trimodal) density distribution of log2 ratios and histogram of merged ratios. The merged level with the most segments assigned to it was chosen as the present level. All segments with this merged value or above were predicted as present. An arrow indicates the minimum log2 ratio for present probes for this particular sample.