# Finding Critical Values In constructing confidence intervals for σ or σ 2 we use Table A-4 to find…

Finding Critical Values In constructing confidence intervals for σ or σ2 we use Table A-4 to find the critical values X2L and X2R but that table applies only to cases in which n ≤ 101, so the number of degrees of freedom is 100 or smaller. For larger numbers of degrees of freedom, we can approximate X2L and X2R by usingwhere k is the number of degrees of freedom and is the critical z score described in Section 7-2. STATDISK was used to find critical values for 189 degrees of freedom with a confidence level of 95%, and those critical values are given in Exercise 17. Use the approximation shown here to find the critical values and compare the results to those found from STATDISK.