When measurements are made on the same experimental item at successive stages of an experiment, the experimental errors include components which are cumulative in nature, and are hence not independent. This dependence may be removed by using incremental changes in the experimental item as the yield variable, and then using a weighted least squares approach to estimate the parameters of the model, assumed to be linear. Alternately, the original cumulative data may be used directly, and Aitken's method of weighted least squares applied to obtain the parameter estimates. These estimates are shown to be identical.