When S2 is the sample variance of a normal random sample
has a chi-squared distribution with n=2 1 df, so
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This suggests that an alternative chart for controlling process variation involves plotting the sample variances and using the control limits
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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
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