internal validity
Summary
Internal validity is how confidently a study shows that the independent variable, rather than some other factor, caused the change in the dependent variable. Random assignment, control groups and control of confounds raise it, and tight control can lower external validity.
Detail
Meaning: Internal validity is the degree to which a study's results can be attributed to the manipulated variable inside that study. If a confounding variable could explain the outcome, internal validity is weak. Common threats include selection bias, attrition (unequal dropout between groups), experimenter bias, demand characteristics, placebo effects, and changes over time such as history or maturation. It is strengthened by random assignment, a control or comparison group, blinding, standardized procedures, and actual manipulation of the independent variable.
On the MCAT: A passage describes a study, and you are asked to spot a flaw or improve the design. Typical flaws are participants choosing their own group (self-selection), one group losing more members than the other, or researchers who know which participants got the treatment. Randomized experiments support causal claims better than correlational or observational designs, which usually have weaker internal validity. Highly controlled lab settings tend to raise internal validity but may lower external validity.
Common confusion: Internal validity is not external validity, which is whether results generalize to other people and settings, and it is not reliability, which is consistency of measurement. A reliable measure can still sit in a confounded study. Random assignment protects internal validity, while random sampling from a population supports generalizability.
Sources
- Psychology
- Research Methods in Psychology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.