Twenty observations on etch uniformity on silicon wafers are taken during qualification experiment for plasma etcher: The data are as follows:
5.34 6.00 5.97 5.25 6.65
7.55 7.35 6.35 4.76 5.54
5.44 4.61 5.98 5.62 4.39
6.00 7.25 6.21 4.98 5.32

a. Construct 95 percent confidence interval estimate of σ²