The extent to which a variable or intervention measures what it is supposed to measure, or accomplishes what it is supposed to accomplish. The internal validity of a study refers to the integrity of the experimental design. The external validity of a study refers to the appropriateness by which its results can be applied to non-study patients or populations.
Observable, measurable (or categorisable) characteristics that vary among cases in a study e.g. height, weight, blood pressure, gender, eye colour and length of hospital stay.
In an experiment, the independent variable is the variable that is varied or manipulated by the researcher, and the dependent variable is the response that is measured. An independent variable is the presumed cause, whereas the dependent variable is the presumed effect.