What is ex post facto design?
From PsychWiki - A Collaborative Psychology Wiki
Ex post facto design is a non experimental research technique in which preexisting groups are compared on some dependent variable, it is a type of study that can masquerade as a genuine experiment. This experiment appears to be a true experiment because of the way the groups are separated and the way the analysis is performed, but it is still subject to the same limitations as non experimental research. The assignment of participants to the levels of the independent variable is based on events that occurred in the past, this is where the name is derived from. This non experimental research is similar to an experiment because it compares two or more groups of individuals with similar backgrounds who were exposed to different conditions as a result of their natural histories.
Lammers, W. J., and Badia, P. (2005). Fundamental of Behavioral Research. California: Thomson and Wadsworth.