Constant: a fixed value. Not a variable.
Variable: a value or characteristic that differs among individuals. It can be described, counted, or measured.
Independent Variable: the variable researchers manipulate in an experiment. Affects the dependent variable(s).
Dependent Variable: the variable affected by the independent variable in an experiment.
Sample: a smaller group selected from a larger group (the population). Researchers study samples to draw conclusions about the larger group.
Population: the entire collection of people, animals, plants, things, etc. researchers wish to examine. Since populations are often too large to include every member in a study, researchers work with samples to describe or draw conclusions about the entire population group using statistical tests.
Participant: an individual who participates in a study. This is a more recent term. (Used with human research.)
Subject: another way to describe a participant. This is a more traditional term. (Used with human and animal research.)
Attrition: loss of participants/subjects in a study.
Reliability: the extent to which a measure or tool yields consistent results on repeated trials.
Validity: the degree to which a study accurately assesses what it is attempting to assess.