Observational Unit — Definition, Formula & Examples
An observational unit is the individual person, animal, object, or event that you collect data on in a study. If you measure something, the observational unit is the thing you are measuring it on.
An observational unit (also called a unit of observation or a case) is an entity on which one or more variables are recorded. In a data table, each observational unit corresponds to exactly one row, and each variable corresponds to one column. In an experiment, the observational unit can differ from the experimental unit, which is the entity that receives a treatment.
Worked Example
Problem: A school tests a new teaching method. The method is randomly assigned to 6 classrooms, and each classroom has 25 students. At the end of the term, every student's test score is recorded. Identify the observational unit and the experimental unit, and find how many observational units there are.
Step 1: Ask what each recorded measurement belongs to. A test score belongs to one student, so the observational unit is a student.
Step 2: Ask what received the treatment. The teaching method was assigned to whole classrooms, so the experimental unit is a classroom. Here the two units differ.
Step 3: Count the observational units. Every student in every classroom has a score.
Answer: The observational unit is a student, and there are 150 of them. The experimental unit is a classroom, and there are 6 of them.
Why It Matters
On the AP Statistics exam, free-response questions often ask you to identify the units in a study before you describe a design or an analysis. Telling observational units apart from experimental units matters. In the example above, the treatment was randomized among only 6 classrooms, not 150 students, and the analysis has to reflect that.
Common Mistakes
Mistake: Naming a variable as the observational unit, such as saying the unit is 'test scores' or 'blood pressure.'
Correction: A variable is a characteristic you record. The observational unit is the thing that has that characteristic. Ask 'Whose score?' or 'Whose blood pressure?' The answer, such as a student or a patient, is the unit.