What is validity? If I say “that’s a valid argument” to you, it means your facts and your logic seem reasonable to me. In research methods, we talk about validity because we want to make statements about the world; we want to make knowledge claims. We want these claims to be valid, meaning they should be well-grounded in logic and fact so that we can trust in them.
Much of scientific research is concerned with making claims about causality. In education research, for example, we want to know what causes students’ math achievement to be high or low. Is it what their teacher does? Their raw brain power? How hard they work? And so forth. Obviously, the answer is, it’s many things, but to what extent possible we want to isolate the factors that are under our control (teaching method, curriculum, school culture) and find the factors that will result in the highest math achievement.
Internal validity = Extent to which you can infer causality
The internal validity of a research study is the extent to which you can make a causal claim based on the study; it is the validity of the causal inference you make. Different research designs provide stronger or weaker internal validity. For example, well-designed randomized experimental designs generally are considered to provide the strongest internal validity. Quasi-experimental studies in which treatments are assigned randomly to intact groups (e.g., classrooms) can have strong internal validity also.