D.3 Identify threats to internal validity (e.g., history, maturation).

Any uncontrolled variables in the treatment setting that could impact the outcomes and/or the effectiveness of an intervention and therefore the internal validity of the experiment.

Any uncontrolled participant-related variables that may interfere with the outcomes of an intervention and therefore the internal validity of an experiment.

Refers to when initial response rates in baseline are weaker due to a subject needing to get warmed up.