An educational psychologist conducts a study to compare two different methods of teaching algebra. Class $X$ is taught using an interactive software package in a modern, air-conditioned computer laboratory during the morning. Class $Y$ is taught using traditional whiteboard methods in a hot, poorly ventilated classroom during the late afternoon. At the end of the term, Class $X$ achieves significantly higher exam scores than Class $Y$. Because the physical environment and time of day differed systematically between the two groups, these extraneous variables acted as confounding variables. This failure to control confounding variables most directly compromises which type of validity?