Comparison of PM and MM measurements within a probe set. Closed and open circles represent PM and MM measures, respectively. (A) High expression level. The PM value is suitably larger than MM value in every probe pair. Separating the signal of specific binding from that of non-specific binding might be possible in such a case. (B) Moderate expression level. The MM value is near the PM value in every probe pair; separating the signal of specific bindings from that of non-specific binding is difficult. However, the value of X is informative as to whether a gene is truly being expressed or not. (B1) shows 'On' (X = 8)) and (B2) shows 'Off' (X = 3). (C) Low expression level. It is difficult to determine whether a gene is expressed or not. (C1) shows 'On' (X = 10) and (C2) shows 'Off' (X = 6).