The document discusses Student's t-test, which is useful for three situations: when sample sizes are small, when the population standard deviation is unknown, and when comparing two samples. It describes how Student's t-test addresses the problems with small sample sizes that violate the Central Limit Theorem. It also explains how the t-test can be used to estimate an unknown population standard deviation from the sample standard deviation. Finally, it provides examples of using a t-test to compare the means of two samples and of using a paired t-test to compare salaries between two cities for the same jobs.