The Selective Control Design Refers to Combining Various Quasi-Experimental Designs

The selective control design refers to combining various quasi-experimental designs to control the threats to internal validity that are thought to be the most viable to a particular evaluation. Suppose that a college instituted a program to help retain freshmen so that a greater proportion re-enrolled for the sophomore year than in the past. The obvious comparison is between the re-enrollment rates of the current freshmen and the previous year's freshmen. What threats to internal validity weaken the conclusions that one could draw from such a comparison? Expand on this basic design, seeking to control at least some of the threats to the internal validity of the design.