Table 1.

Threats to Internal Validity

1. Ambiguous temporal precedence: Lack of clarity about whether intervention occurred before outcome

2. Selection: Systematic differences over conditions in respondent characteristics that could also cause the observed effect

3. History: Events occurring concurrently with intervention could cause the observed effect

4. Maturation: Naturally occurring changes over time could be confused with a treatment effect

5. Regression: When units are selected for their extreme scores, they will often have less extreme subsequent scores, an occurrence that can be confused with an intervention effect

6. Attrition: Loss of respondents can produce artifactual effects if that loss is correlated with intervention

7. Testing: Exposure to a test can affect scores on subsequent exposures to that test

8. Instrumentation: The nature of a measurement may change over time or conditions

9. Interactive effects: The impact of an intervention may depend on the level of another intervention

Adapted from Shadish et al.⁴