A choice of methods exists for determining the variability of an averaged wavefront measurement. Pixel-by-pixel standard deviation (SD) maps provide a spatial description of the variability as well as a scalar magnitude. An efficient algorithm for computing SD maps as measurements occur is given. Results for simulated and experimental wave-front maps are shown. Plots of the average SD as a measurement progresses can be used to characterize a test system and determine the minimum number of measurements required for acceptable results.