Observational unit

Naming the observational unit first is the habit that keeps a study straight: the unit is the who or the what, and the variables are what you record about it. For example, in a study of 200 cars the observational unit is one car, while make is a categorical variable recorded on it and fuel economy in miles per gallon is a quantitative one. Get the unit wrong and every count after it is wrong, since 40 schools and 40,000 students are different data sets with different sample sizes. In an experiment, the observational unit a treatment is assigned to is called the experimental unit, or a subject when the units are people.