Finding Critical Values In constructing confidence intervals for s or s2, we use Table A-4 to find the critical values and , but that table applies only to cases in which n # 101, so the number of degrees of freedom is 100 or fewer. For larger numbers of degrees of freedom, we can approximate and by using where k is the number of degrees of freedom and is the critical z score first described in Section 6-2. Construct the 95% confidence interval for s by using the following sample data: The measured heights of 772 men between the ages of 18 and 24 have a standard deviation of 2.8 in. (based on data from the National Health Survey).