The one-sample t test is one of three members of the t test family that is discussed in this book. In an earlier chapter, it was learned that the z-test requires knowing the value of the population standard deviation, information that is in fact not usually known. Thus, instead of calculating the population deviation directly, the t-statistic estimates the population standard deviation by using a correction factor in the formula. Otherwise, the same four-step hypothesis testing procedure is used as previously learned to test a hypothesis about a population mean based on the mean of a sample. The characteristics of the t-statistic and its similarities and differences from the normal distribution are examined. Cohen’s d effect size as applied to the t-statistic is also discussed. Excel instructions are provided for calculating the statistics required for a one-sample t test, including those necessary for both obtained and critical t-values.