# Concentrations

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10.48 Nitrogen and Seagrass. Refer to Exercise 10.42.

a. Determine a 98% confidence interval for the difference, M1-M2, between the mean sediment ammonium concentrations in CCB and LLM.

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10.48 Nitrogen and Seagrass. Refer to Exercise 10.42.

a. Determine a 98% confidence interval for the difference, M1-M2, between the mean sediment ammonium concentrations in CCB and LLM.

b. Interpret your answer in words. Note: You must use Z a/2 or T a/2 = 2.349 to determine the 98% confidence level.

10.66 Measuring Treadwear. Stichler, Richey, and Mandel compared two methods of measuring treadwear in their paper "Measurement of Treadwear of Commercial Tires" (Rubber Age, Vol. 73:2) Eleven tires were each measured for treadwear by two methods, one based on weight and the other on groove wear. The following are the data, in thousands of miles.

Weight Method Groove Method Weight Method Groove Method

30.5 28.7 24.5 16.1

30.9 25.9 20.9 19.9

31.9 23.3 18.9 15.2

30.4 23.1 13.7 11.5

27.3 23.7 11.4 11.2

20.4 20.9

At the 5% significance level, do the data provided sufficient evidence to conclude that, on average, the two measurement methods give different results?

10.72 Measuring Treadwear. Refer to Exercise 10.66. Determine a 95% confidence interval for the mean difference in measurement by the weight and groove methods. Interpret your answer.

10.76 Suppose that you want to perform a hypothesis test to compare the means of two populations, using a paired sample. For each part, decide whether you would use the paired t-test, the paired Wilcoxon signed signed-rank test, or neither of these test, if preliminary data analyses of the sample of paired differences suggest that the distribution of the paired-difference variable is

a. approximately normal.

b. highly skewed; the sample size is 20.

c. symmetric bimodal

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