Cardiovascular psychophysiologists often express variability in successive data as the root of the mean squared successive differences. Other parameters of variability, such as the standard deviation and the coefficient of variation, are largely determined by slow (such as circadian) rhythms or linear trends. When the parameters of variability in blood pressure data are calculated each hour over an increasing period of time (to simulate the termination of the blood pressure monitoring after a variable duration), standard deviations and coefficients of variation increase due to the decrease in blood pressure levels at evening and night, but the root of the mean squared successive differences does not. The advantage of using the root of the mean squared successive differences as a parameter of blood pressure variability is shown in data from 147 outpatients with 10784 blood pressure readings.