The Critical Difference: The difference from the base value (1 sample) or the difference between (2 samples) that the experimenter determines critical to detect (with 1 - b confidence). For means and proportions, the critical difference is a straight difference in values (although for means tests, what is really important is the expected standard deviation). For standard deviations and rates, the critical difference is usually a ratio corresponding difference relative to the to a fractional increase or decrease.