For sample mean x = 80, sample size n = 36 and sample standard deviation s = 24 find 90% confidence interval for population mean. This is the case when population standard deviation is not known. Use formula on page 328 with t-value. Find t-value in Appendix Table IV. Tip: For confidence level 90% α = 1 - 0.90 = 0.10, α/2 = 0.10/2 = 0.05 So, in Table IV use column t0.05 and line with df = n - 1 = 36 - 1 = 35.