In Example 4.3, the paper mill had to demonstrate that its effluent had mean DO greater than 6 mg/L. But to ensure against occasional very low DO, keeping the mean high is not enough, the mill must also keep the standard deviation low.

(a) Using the data in Table 4.3, is there evidence that the standard deviation is less than 0.5 mg/L? Use a significance level of 1%.

(b) The test used in

(a) requires the data come from a normal distribution. Use a box plot and a normal probability plot to check this assumption.

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