Causality and control: threats to internal validity.

The essence of experimental research is to establish causal relationships between variables and this requires internal validity. An experiment is said to have internal validity when extraneous variables have been controlled. However, there are many threats to internal validity. This article considers 13 of them, providing examples which clarify and help to identify under what conditions these threats may occur. The threats discussed are history, maturation, testing, instrumentation, statistical regression, selection mortality, interactions with selection ambiguity about the direction of causal influence, diffusion of imitation of treatments, compensatory equalization of treatments, compensatory rivalry by respondents receiving less desirable treatments and resentful demoralization of respondents receiving less desirable treatments.