This chapter examines the first parametric test, Student's t test. It is designed to determine whether or not the means of two samples of data are significantly different. The data must be on an interval or ratio scale and the samples must be drawn from populations where the data are normally distributed. The t test investigates whether the differences between means observed in the samples of data indicate that these samples come from different populations or whether the differences are only chance differences and the samples come from the same population. Besides using the t test to determine whether a given sample was drawn from a given population, it can also be used to determine whether two samples were drawn from the same population or from different populations. The related-samples t test is similar in approach to the single-sample t test. The t test can be completely replaced by the more general analysis-of-variance procedure.