Variation inherent to each session in any recording situation has to be mastered to obtain informative mean values and differences. Variation due to time trend and unsteady consecutive readings versus inter-arm variations in blood pressure (BP) in the operator-initiated, self-initiated, and sleep recordings were studied by oscillometry. They were small in operator-initiated recordings, increased in self-recordings, and were paramount in sleep. A passive supine patient and simultaneous recordings with cuffs placed in equal horizontal positions are essential conditions for the estimation of a real mean inter-arm difference. The relative variation of pulse pressure (PP), compared with other components, was largest within each situation. Most of the systolic BP (SBP)/diastolic BP (DBP) variation in sleep was due to unequal vertical cuff distances to the heart level. Only sleep PP was independent of the vertical position of cuffs. A plot of inter-arm differences can visualize systematic errors in sequences of sleep SBP/DBP. As is well known, every doubling of variation (SD) requires quadrupling the number of independent recordings in order to retain the same size of confidence interval (CI) for a mean value.