The article explains how to analyze data from related groups, like before and after a treatment. For data that follows a normal distribution and has only two time points, a paired t-test is used. If there are more than two time points, a repeated measures analysis of variance is appropriate. For skewed or ordinal data, a Wilcoxon signed-rank test or Friedman's test can be used. These methods help researchers understand changes over time in things like glucose levels or pain scores.