Refer to Exercise 19.

Use the more accurate normal approximation to estimate the critical values X2 0.025 and X2 0.975 for a 95% confidence interval, and construct a 95% confidence interval for σ.

A more accurate normal approximation to X2 α is given by

where zα is the z-score that has area α to its right.

Exercise 19

A sample of size 101 from a normal population has sample standard deviation s = 40.

Use Table A.4 to find the exact critical values X2 0.025 and X2 0.975 for a 95% confidence interval, and construct a 95% confidence interval for σ.

x 0.5 (za + 2k - 1