The effect of measurement error on the power of a control chart is examined for the case where both the process average and variance can change. Expressions for the power of theX̄andRcharts are derived. It is shown that if the measurement error is relatively large in comparison to the inherent process error, then the power of theX̄-Rchart is reduced and the sample size should be increased to recover the lost power. The sample size needed to obtain a desired power level can be calculated.