Threats to Internal Validity (MRS. SMITH)

  1. Maturation: physiological processes that count account for changes in the behavior due to simple passage of time. For instance, the aging process of simply growing older and the developmental changes associated, or physiological states like hunger, fatigue, attention span, or motivation that change over time naturally.

  2. Regression to the mean: the tendency that participants who receive extreme scores when tested tend to have less extreme scores on subsequent retesting even in the absence of treatment effects.

  3. Selection of subjects: Any bias in selecting and assigning participants to groups that results in systematic differences between the participants in each group.

  4. Selection by maturation interaction: The treatment and no-treatment groups, although similar at one point, would have grown apart (i.e., developed differently) even if no treatment had been administered. Even though pretest scores may have been the same, groups that are not matched as well on other relevant variables may cause the groups to naturally become different after a period of time.

  5. Mortality: Differential dropping out of some subjects from the comparison groups before the experiment is finished, resulting in differences between the groups that may be unrelated to the treatment effects. The subjects who drop out may be different than those who complete the treatment, which may inflate, obscure, or confuse the treatment effects of interest. Researcher’s exclusion criteria may also cause a mortality bias.

  6. Instrumentation: Changes in the measurement procedures that may result in differences between the comparison groups that are confused with the treatment effects.

  7. Testing: When participants are repeatedly tested, changes in test scores may be due to practice or knowledge about the test procedure gained from earlier experiences rather than any treatment effects. Similar to maturation except that the change is caused by the treatment procedure itself.

  8. History: Extraneous events occurring during the course of the experiment that may affect the participants’ responses on the dependent measure. Examples may be major events in society or minor events within the experimental situation.