Ambiguous Temporal Precedence
Lack of Clarity about which variable occurred first may yield confusion about which variable is the cause and which is the effect.
Systematic differences over conditions in respondent characteristics that could also cause the observed effect.
Events occurring concurrently with treatment could cause the observed effect.
When units are selected for their extreme scores, they will often have less extreme scores on other variables, an occurrence that can be confused with a treatment effect.
Loss of respondents to treatment or to measurement can produce artifactual effects if that loss is systematically correlated with conditions .
Exposure to a test can affect scores on subsequent exposures to that test, an occurrence that can be confused with a treatment effect.
The nature of a measure may change over time or conditions in a way that could be confused with a treatment effect.
Additive and Interactive Effects of Threats to Internal Validity
The impact of a threat can be added to that of another threat or may depend on the level of another threat.
Naturally occuring changes over time could be confused with a treatment effect.