What threat to internal validity arises if different measures are used for pre-test and post-test observations?

The correct answer is instrumentation. In the context of a study, instrumentation refers to the potential threat to internal validity that occurs when different methods or tools are used to collect data at different points in time—in this case, for the pre-test and post-test observations. When different measures are applied, it can create disparities in what is being assessed, leading to invalid conclusions about the effects of an intervention.

For instance, if a researcher uses a standardized assessment for the pre-test but opts for a different, perhaps less reliable, measure for the post-test, any observed differences may reflect variances between the two instruments rather than true changes in the study participants. This undermines the ability to accurately attribute changes in outcomes to the treatment or intervention being evaluated.

Understanding instrumentation as a threat is crucial since it points to the importance of consistency in measurement tools throughout a study to ensure that any observed effects are genuinely reflective of the interventions rather than artifacts of differing assessment techniques. Keeping measures consistent allows for a clearer interpretation of data and strengthens the study's overall validity.