Research key concepts

The independent variable is the major variable which you hope to investigate. It is the variable which is selected, manipulated, and measured by the researcher.

Internal validity refers to how well an experiment is done, especially whether it avoids confounding (more than one possible independent variable [cause] acting at the same time). The less chance for confounding in a study, the higher its internal validity is.

Therefore, internal validity refers to how well a piece of research allows you to choose among alternate explanations of something. A research study with high internal validity lets you choose one explanation over another with a lot of confidence, because it avoids (many possible) confounds.

Internal validity is a form of experimental validity. It refers to the validity of the study itself. It  means that no variable cause the results except for the one you are studying. According to Ehow contributor Lisa Miller, in a study which contain high internal validity, "the outcomes can confidently be said to directly result from the study's manipulation. Problems related to selection, bias, maturation and attrition, among others, can negatively affect a design's internal validity".

The internal validity of a research study is the extent to which the outcome is a function of the factor you have selected rather than other factors you haven't controlled.