[en] The experimentally determined relationship between the level of liquid and volume of liquid in a tank can be used to obtain volume estimates that correspond to liquid-level measurements. Several calibration experiments (runs) are made to estimate the calibration equation. The calibration equation is used to estimate the quantity of liquid transferred from a tank between measurement periods. Difficulties can arise when run-to-run differences are large relative to the precisions of liquid-level and volume measurements. This paper addresses the seldom-discussed but important problem of combining and analyzing data from two or more calibration runs. Emphasis is placed on exploratory methods such as diagnostic plots that are compatible with applicable statistical models. 10 references, 3 figures, 2 tables