Platelet Counts of Women 99% confidence; n = 147, s = 65.4. 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.