Variables are the things you measure on your subjects, which can be humans, animals, or cells. Variables can represent subject characteristics, i.e. weight, height, sex, the things you are really interested in i.e. stress levels, rate or injury, physiological, psychological or sociological variables, and variables representing the timing of measurements and nature of any treatments subjects receive i.e. before and after a real drug or a placebo.

To quantify the relationships between these variables, we use values of effect statistics such as the correlation coefficient, the difference between means of something in two groups, or the relative frequency of something in two groups.