Signal of individual probes and dependence on present versus absent RNA molecules. (a, b) Plot of probe-pair signals for the three C chips, highlighting (a) the empty probe pairs or (b) the present probe pairs in green. (c) Receiver-operator characteristic (ROC) curves at the probe-pair level for several absent/present metrics. The metric (PM - MM)/(PM + MM) gives the same result as the green curve. (d)Receiver-operator characteristic curves at the probe-set level for several absent/present metrics combined using the Wilcoxon rank sum test.