When indirect measurements are used to obtain rate-of-change data, an additional source of uncertainty is introduced that must be taken into account in the mean-rate and standard-error estimates. A procedure to account for this uncertainty is described. The chain rule of differentiation is used to decompose the rate into the product ofcalibration and primary experimentparts. Mean-rate and standard-error estimates are then obtained in terms of the component means and standard errors. The procedure is demonstrated by an analysis of some fatigue-crack-growth data.