The performance of a process control algorithm based on the EWMA stati
stic is analyzed. A simple condition relating the EWMA weight and the
estimated process gain is shown to ensure that the control strategy is
stable for a first-order multiple-input, single-output process that m
ay be drifting or wandering. A similar condition is shown to guarantee
stability far a second-order, single-input process. An expression is
derived for the output mean squared deviation from target as a functio
n of the algorithm parameters, how quickly the process drifts or wande
rs, and the amount of noise. The results are illustrated in the contex
t of an epitaxial growth process.