The computation times for the scanning and VS model observers in this study were disparate. Recall that the channel covariance matrices for the CH observer were computed (but not inverted) prior to the study. Despite this preprocessing, the CH computation times were relatively high: in IDL (Interactive Data Language, Exelis Visual Information Solutions, Boulder, Colorado) to read 450 test images compared to less than a minute for the VS and CNPW observers. These timings are for the model observers without internal noise or search thresholding. The lesion search region for the model observers contained 380 locations. From this, the VS observer averaged between 20 and 40 focal points per image, with the higher numbers associated with lower collimator sensitivities. However, the code implementation of the candidate analysis did not exploit this search reduction.