The compressive strength for nine samples in dryconditions on the twenty-eighth

Question

The compressive strength for nine samples in dryconditions on the twenty-eighth day are as follows (inmegapascals):

40.2 30.4 28.9 30.5 22.4

25.8 18.4 14.2 15.3

b) Find a 98% two-sidedconfidence interval onmean compressive strength. Provide a practical interpretation ofthis interval and explain why the lower end-point of the intervalis or is not the same as in part(a).

c) Find a 99% upper one-sidedconfidence interval onthe variance of compressive strength. Provide a practicalinterpretation of this interval.

d) Find a 98% two-sidedconfidence interval onthe variance of compression strength. Provide a practicalinterpretation of this interval and explain why the upper end-pointof the interval is or is not the same as in part(c).

Explanation / Answer

n = 9

mean , = 25.12

standard deviation , = 8.42

standard error, e = / n = 8.42 / 9 =2.81

a.) for 99% one sided confidence, z = 2.33

a 99% lower one-sided confidence interval of the meancompressive strength = - ze

= 25.12 - 2.33 x 2.81 = 18.5727

= 25.12 + 2.33 x 2.81 = 31.6673

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