When the averages control chart is applied to monitor a manufacturing process, three parameters should be determined, sample size, sampling interval between successive samples and the control limit for the chart. This study shows how the effect of process variability caused by common and assignable causes on the values of averages control chart parameters. An example is presented and then based on this example, sensitivity analysis is performed to show the direction of control chart parameters changes in the presence of changes in the magnitude and frequency of process shift and the costs of discovering and correcting the causes of these shifts