find the 95% confidence interval for the variance of heights of maple trees if a sample of 21trees has a standard deviation of 14.2 feet. Solution Given a=0.05, chisquare with 0.025, df=n-1=20 is 9.59 (check chisquare table) chisquare with 0.975, df=n-1=20 is 34.17 (check chisquare table0 So 95% CI is ((n-1)*s/34.17, (n-1)*s/9.59) --> (20*14.2/34.17, 20*14.2/9.59) --> (8.311384, 29.61418).