Question

. An article in the Journal of Engineering Manufacture, 2002, reports that in a sample of 64 micro-drills drilling a low-carbon alloy steel, the average lifetime (expressed as the number of holes drilled before failure) was 12.68 with a standard deviation of 6.40 holes.

a)Find a 95% confidence interval for the mean lifetime of micro-drills under these conditions

b)Based on the micro-drill lifetime data presented in problem #7, an materials engineer reported a confidence interval of (11.09, 14.27); however, the engineer neglected to specify the confidence level. What is the level of this confidence interval?

Answer 1

(a)

Answer: (11.08, 14.28)

b)

The width of confidence interval is

14.27 - 11.09 = 3.18

So margin of error is

ME = 3.18 / 2 = 1.59

The critical value of t used for making confidence interval is

So area right to score 1.988 is 0.0256 that is

The required confidence interval is