內在效度

Internal validity is the validity of (causal) inferences in scientific studies, usually based on experiments as experimental validity.

In scientific experimental settings, researchers often manipulate a variable (the independent variable) to see what effect it has on a second variable (the dependent variable).

For example, a researcher might, for different experimental groups, manipulate the dosage of a particular drug between groups to see what effect it has on health.

In this example, the researcher wants to make a causal inference, namely, that different doses of the drug may be held responsible for observed changes or differences. When the researcher may confidently attribute the observed changes or differences in the dependent variable to the independent variable, and when he can rule out other explanations (or rival hypotheses), then his causal inference is said to be internally valid.

內在效度：
當我們相信「差異」或「相關」的確存在時，接下來就可以問「差異是誰造成的？」是「獨變項」？還是另有其它因素呢？「內在效度」就是指實驗者對所操弄的「獨變項」對「依變項」造成影響的信心程度。當我們越有信心排除與獨變項無干的其它競爭因素，實驗的內在效度就越高。