Throughout the course, we will be using many of the terms introduced in this lesson. Let's start by defining some of the most frequently used terms: case, variable, and constant.
A case is an experimental unit. These are the individuals from which data are collected. When data are collected from humans, we sometimes call them participants. When data are collected from animals, the term subjects is often used. Another synonym is experimental unit.
A variable is a characteristic that is measured and can take on different values. In other words, something that varies between cases. This is in contrast to a constant which is the same for all cases in a research study.
- An experimental unit from which data are collected
- Characteristic of cases that can take on different values (in other words, something that can vary)
- Characteristic that is the same for all cases in a study
Let's look at a few examples.
Example: Study Time & Grades Section
A teacher wants to know if third grade students who spend more time reading at home get higher homework and exam grades.
The students are the cases. There are three variables: amount of time spent reading at home, homework grades, and exam grades. The grade-level of the students is a constant because all students are in the third grade.
Example: Dog Food Section
A researcher wants to know if dogs who are fed only canned food have different body mass indexes (BMI) than dogs who are fed only hard food. They collect BMI data from 50 dogs who eat only canned food and 50 dogs who eat only hard food.
The cases are the dogs. There are two variables: type of food and BMI. A constant would be subspecies, because all cases are domestic dogs.
Example: Age & Weight of Sea Otters Section
Researchers are studying the relationship between age and weight in a sample of 100 male sea otters (Enhydra lutris).
The 100 otters are the cases. There are two variables: age and weight. Biological sex is a constant because all subjects are male. Species is also a constant.