Experimental Unit vs Observational Unit
Both terms below come up in the same part of the course, and students mix them up. Here is each one defined on its own, side by side, so you can see where they part company.
Experimental unit
Collecting data and study design
An experimental unit is the individual or object a treatment is assigned to; when the units are people they are called subjects or participants.
The experimental unit is whatever gets randomly assigned, and naming it correctly sets how many units the study really has. For example, if a teacher assigns one review method to each of 6 whole classes, the experimental units are the 6 classes, not the 150 students inside them. Calling the students the units would inflate how much evidence you think you have. Keep it separate from the observational unit, which is whatever a measurement gets recorded on: here the experimental unit is the class, while the response is recorded for each of the 150 students.
Observational unit
Variables and data types
An observational unit is the item or individual from which a datum is collected, that is, one row of the data set.
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.