When S2 is the sample variance of a normal random sample has a chi-squared distribution with n=2 1 df, so
This suggests that an alternative chart for controlling process variation involves plotting the sample variances and using the control limits
Construct the corresponding chart for the data of Exercise 11
Exercise 11 The accompanying table gives sample means and standard deviations, each based on n = 6 observations of the refractive index of fiber-optic cable. Construct a control chart, and comment on its appearance