If you have a continuous measurement and two groups you’d like to compare based on that measurement, what’s the first statistical test that comes to mind? Chances are it’s the two-sample t-test, sometimes known as Student’s t-test. It’s typically the first statistical test taught in an introductory statistics course, it’s well known and understood, and it has good theoretical properties—so if a t-test answers your research question, you should probably use it.

Data that has some kind of hierarchical structure to it is very common in many fields, but is rarely discussed in introductory statistics courses. Terms used to describe this kind of data include hierarchical data, multi-level data, longitudinal data, split-plot designs or repeated measures designs. Statistical models used for these types of data include mixed-effects models (often abbreviated to just mixed models), repeated measures ANOVA and generalised estimating equations (GEEs).

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