Mean group CBF displayed as a function of the frequency (i.e., number of subjects) in which voxels are labeled as WMH. Each circle represents a single voxel. For example, a single circle in Frequency = 5 represents the total group mean CBF for a voxel that is labeled as WMH in five subjects. Note subtle decline in group mean CBF across frequency (red dot) in marked decrease in variability, suggesting that areas with consistently lower CBF are most likely to be WMH across individuals. The number of voxels included for each frequency, 1 through 15, respectively, are 12109, 3811, 1796, 1125, 795, 608, 482, 377, 269, 212, 114, 58, 32, 10, and 4.