When S2 is the sample variance of a normal random sample, has a chi-squared distributio...

When S² is the sample variance of a normal random sample, has a chi-squared distribution with n – 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. [Hint: The lower- and upper-tailed chi-squared critical values for 5 df are .210 and 20.515, respectively.]

Reference 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. and