Ex Post Facto Design

An ex post facto design is a type of research study in which groups of participants are determined by pre-existing conditions and events from the past. Latin for 'after the fact', in ex post facto designs the groups are compared with each other on a dependent variable (like an experimental design) but it is considered a quasi-experimental design because the independent variable is not manipulated.

The independent variable condition is based on pre-existing conditions instead of random assignment. Groups are based on subject variables that are already present in the participants.

For example, researchers are interested in the drink choices of Type 2 diabetics. So they form two groups, a group formed of Type 2 diabetes sufferers and a group with no diagnosed diabetics. The researchers then allow them to choose whatever drink they want from many choices in a waiting room before the study 'begins'. They then record what each person chooses and analyze the results.