`pairwise_tukeyhsd()`, Ch.20 section 20.8 Tuskegee syphilis study, Ch.4 section 4.7, **Ch.27 section 27.6** Tversky, Amos, Ch.8, Ch.9 two-proportion z-test, **Ch.16 section 16.6** two-sample t-test, **Ch.16 section 16.3** two-tailed test, **Ch.13 section 13.8** Type I error, **Ch.13 section 13.9**,