The term ex post facto is from Latin and literally means “from after the action.” This design, when employed experimentally, functions as a comparison [Page 469]between groups without the use of a pretest. The design type is employed both by classic experimental research as well as field research. Experimental research involves comparing conditions often employing random assignment to groups whereas field research often employs naturally occurring groups (e.g., classes, locations). The assumption is that the groups ultimately compared are groups that began at the same point and the difference between the groups represents only the difference in experience caused by an intervention or some other occurrence. This entry examines ex post facto designs and their limitations.