Consider a study to evaluate the effect on college student grades of dorm room Internet connections. In a large dorm, half the rooms are randomly wired for high-speed Internet connections (the treatment group), and final course grades are collected for all residents. Which of the following pose threats to internal validity, and why?
a. Midway through the year all the male athletes move into a fraternity and drop out of the study (their final grades are not observed).
b. Engineering students assigned to the control group put together a local area network so that they can share a private wireless Internet connection that they pay for jointly.
c. The art majors in the treatment group never learn how to access their Internet accounts.
d. The economics majors in the treatment group provide access to their Internet connection to those in the control group, for a fee.