The process of calibrating a small number of 'unknown' standards relative to one or two reference standards involved determining differences among the group of objects. Drift, due most often to temperature effects, or a 'left-right' polarity effect can bias both the values assigned to the objects and the estimate of the effect of random errors. This note presents schedules of measurements of differences that eliminate the bias from these sources in the assigned value and variances at the same time gives estimates of the magnitude of these extraneous components. The use of these designs in measurement process control is discussed and a computer program in BASIC is presented. (Author).