Classification of Variables

Between-subjects Variable Values of the variable are examined as they are observed to change from one individual to another.  When a variable is administered Between-Subjects, subjects each receive ONE level of the Independent Variable

Within-subject Variable Values of the variable are examined as they are observed to change from time to time within an individual.  When a variable is administered Within-Subject, subjects receive ALL levels of the Independent Variable (also called Repeated Measures).

Qualitative Variable Composed of categories which are not comparable in terms of magnitude

Quantitative Variable Can be ordered with respect to magnitude on some dimension  

Continuous Variable A quantitative variable which can be measured with an arbitrary degree of precision. Any two points on a scale of a continuous variable have an infinite number of values in between

Discrete Variable A quantitative variable where values can differ only by well-defined steps with no intermediate values possible.

The engine in your car can have 3 cylinders or 4 cylinders but it can't have 3.1 cylinders. Number of cylinders is a discrete variable: between two adjacent values (e.g. 3 and 4 cylinders) on the scale there cannot be any intermediate value (e.g. 3.5 cylinders). By contrast, your engine can get 35 or 36 miles per gallon, or it can get 35.1 mpg, or 35.11 or 35.111 or 35.1111... Miles per gallon is a continuous variable: no matter how close two measurements are, it is always possible for a third measurement to fall between them.  

Quantitative variables ask How many? (discrete) or How much? (continuous), while qualitative variables ask Which?

Independent Variable is the variable the investigator wishes to manipulate in order to determine its effects upon behavior.

In an experimental design, each IV is said to have levels. A given IV could conceivably have an infinite number of possible values. We don’t present the IV at all possible values. Those specific values chosen for the IV are referred to as the “Levels of the Independent Variable”.

Dependent Variable is the measurable aspect of behavior the investigator is interested in determining the effect of the manipulation upon.