A researcher selecting only intact classrooms from one high school is concerned about which threat to external validity?

The concern related to selecting only intact classrooms from one high school is primarily associated with selection treatment interaction. This concept refers to the extent to which the characteristics of the selected sample may limit the generalizability of the study's findings to other populations or settings. In this case, by focusing solely on classrooms within a single high school, the researcher may inadvertently introduce a bias that does not reflect the broader educational context, as the students in those classrooms might share similar socio-economic, demographic, or educational backgrounds.

When studying the effectiveness of an intervention, it is crucial to consider how the specific characteristics of the chosen sample may influence the outcomes. If the high school is not representative of other schools, the results may not be applicable to different educational settings or populations, thereby affecting the external validity of the study.

Addressing or acknowledging selection treatment interaction helps researchers better understand the limitations of their findings and consider the applicability of their research beyond the specific sample studied.