Variables are properties or characteristics of some event, object, or person that can take on different values or amounts (as opposed to constants such as pp which do not vary). When conducting research, experimenters often manipulate variables. For example, an experimenter might compare the effectiveness of four types of antidepressants. In this case, the variable is the "type of antidepressant". In general the independent variable is manipulated by the experimenter and its effects on the dependent variable are measured.

An important distinction between variables is between qualitative and quantitative variables. The values of a qualitative variable do not imply a numerical ordering. Values of the variable "religion" differ qualitatively; no ordering of religions is implied. Qualitiative variables are sometimes referred to as categorical variables. Values on qualitative variables do not imply order, they are simply categories.

In the study on the effect of diet discussed above, the independent variable was type of supplement: none, strawberry, blueberry, and spinach. The variable "type of supplement" is a qualitative variable; there is nothing quantitative about it. In contrast, the dependent variable "memory test" is a quantitative variable since memory performance was measured on a quantitative scale (number correct).

Variables such as number of children in a household are called discrete variables. Other variables such as "time to respond to a question" are continuous variables. Of course, the practicalities of measurement preclude most measured variables from being truly continuous.