When translated literally, ex post facto means ‘from what is done afterwards’. In the context of social and educational research the phrase means ‘after the fact’ or ‘retrospectively’ and refers to those studies which investigate possible cause-and-effect relationships by observing an existing condition or state of affairs and searching back in time for plausible causal factors. In effect, researchers ask themselves what factors seem to be associated with certain occurrences, or conditions, or aspects of behaviour. Ex post facto research, then, is a method of teasing out possible antecedents of events that have happened and cannot, therefore, be engineered or manipulated by the investigator. The following example will illustrate the basic idea. Imagine a situation in which there has been a dramatic increase in the number of fatal road accidents in a particular locality. An expert is called in to investigate. Naturally, there is no way in which she can study the actual accidents because they have happened; nor can she turn to technology for a video replay of the incidents. What she can do, however, is attempt a reconstruction by studying the statistics, examining the accident spots, and taking note of the statements given by victims and witnesses. In this way the expert will be in a position to identify possible determinants of the accidents. These may include excessive speed, poor road conditions, careless driving, frustration, inefficient vehicles, the effects of drugs or alcohol and so on. On the basis of her examination, she can formulate hypotheses as to the likely causes and submit them to the appropriate authority in the form of recommendations. These may include improving road conditions, or lowering the speed limit, or increasing police surveillance, for instance. The point of interest to us is that in identifying the causes retrospectively, the expert adopts an ex post facto perspective.
Kerlinger (1970) has defined ex post facto research more formally as that in which the independent variable or variables have already occurred and in which the researcher starts with the observation of a dependent variable or variables. She then studies the independent variable or variables in retrospect for their possible relationship to, and effects on, the dependent variable or variables. The researcher is thus examining retrospectively the effects of a naturally occurring event on a subsequent outcome with a view to establishing a causal link between them. Interestingly, some instances of ex post facto designs correspond to experimental research in reverse, for instead of taking groups that are equivalent and subjecting them to different treatments so as to bring about differences in the dependent variables to be measured, an ex post facto experiment begins with groups that are already different in some respect and searches in retrospect for the factor that brought about the difference. Indeed Spector (1993:42) suggests that ex post facto research is a procedure that is intended to transform a non-experimental research design into a pseudo-experimental form.
Two kinds of design may be identified in ex post facto research—the co-relational study and the criterion group study. The former is sometimes termed ‘causal research’ and the latter, ‘causal-comparative research’. A co-relational