Internal Validity

As explained in the 1960s by Donald Campbell and Julian Stanley in their seminal book Experimental and Quasi-Experimental Designs for Research, internal validity refers to the extent to which the methodological research design used by a researcher can provide empirical evidence to test the possible cause-and-effect relationship between an independent variable (the antecedent), X, and a dependent variable (the consequence), Y. Without adequate internal validity, researchers may offer logical, reasoned arguments to speculate about the possible causal nature of any correlational relationship they observe between X and Y in their data, but they cannot use the internal strength of their research design to bolster such reasoning. Thus, although researchers can (and routinely do) draw casual inferences about "X causing Y" based on speculation from the ...