In a study of the effect of cooling rate on the hardness of welded joints, 50 welds cooled at a rate of 10°C/s had an average Rockwell (B) hardness of 91.1 and a standard deviation of 6.23, and 40 welds cooled at a rate of 30°C/s had an average hardness of 90.7 and a standard deviation of 4.34.

Question

a. Find a 95% confidence interval for the difference in hardness between welds cooled at the different rates.
b. Someone says that the cooling rate has no effect on the hardness. Do these data contradict this claim? Explain.

Answer 1

(a)



(b) No. Since 0 is in the confidence interval, it may be regarded as being a plausible value for the mean difference in hardness.