Nicotine in Menthol Cigarettes 95% confidence; n = 25, s = 0.24 mg. Finding Critical Values and Confidence Intervals. In Exercises 5–8, use the given information to find the number of degrees of freedom, the critical values X2 L and X2 R, and the confidence interval estimate of S. The samples are from Appendix B and it is reasonable to assume that a simple random sample has been selected from a population with a normal distribution.