Tablet hardness. Exercise 26.10 concerns process control data on the hardness of tablets (measured in kilograms) for a pharmaceutical product. Table 26.4 gives data for 20 new samples of size 4, with the x and s for each sample. The process has been in control with mean at the target value μ = 11.5 kg and standard deviation

σ = 0.2 kg

(a) Make both x and s charts for these data based on the information given about the process.

(b) At some point, the within-sample process variation increased from σ = 0.2 kg to σ = 0.4 kg. About where in the 20 samples did this happen? What is the effect on the s chart? On the x chart?

(c) At that same point, the process mean changed from μ = 11.5 kg to μ = 11.7 kg. What is the effect of this change on the s chart? On the x chart?
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