If a relationship is observed between the treatment and the outcome, we must make sure that it is a causal relationship, and that it is not a result of a factor of which we have no control or have not measured. In other words that the treatment causes the outcome (the effect). Threats to internal validity concern issues that may indicate a causal relationship, although there is none. Factors that impact on the internal validity are how the subjects are selected and divided into different classes, how the subjects are treated and compensated during the experiment, if special events occur during the experiment etc. All these factors can make the experiment imitate a behaviour which is not due to the treatment but to the disturbing factor. Examples: History, Maturation, Testing, Instrumentation, Statistical regression, Selection, Mortality, Ambiguity about direction of causal influence, Interactions with selection, Diffusion of imitation of treatments, Compensatory equalisation of treatments, Compensatory rivalry, Resentful demoralization.

Wohlin, C., Runeson, P., Höst, M., Ohlsson, M., Regnell, B., & Wesslén, A. (2000). Introduction to Experimentation in Software Engineering. [-]