Problem
Mario is conducting a study to examine the effect that music has on academic engagement when it is added to online learning modules. As part of his intervention, he adds a soundtrack of Pachelbel's Canon in D Major that plays when Dr. Clark's written lecture on Qualitative Analyses is opened. He finds that students are indeed more engaged when the soundtrack is added. In an attempt to replicate these findings, he adds Orff's O Fortuna to Dr. Clark's lecture on Descriptive and Correlational Research, but does not find the same effect on engagement. What threat to validity should Mario be most concerned with?