Quasi-Experimental Research

• Quasi-experimental designs can seem like a real experiment, but they lack one or more of its essential elements, such as manipulation of antecedents or random assignment to treatment conditions.
• In correlational studies, we are looking for relationships or associations between variables, whereas in quasi-experiments, we are comparing different groups of subjects looking for differences between them, or we are looking for changes over time in the same group of subjects.
• Ex post facto Explores characteristics, behaviors, or effects of naturally occurring events in preexisting groups of subjects.
• Nonequivalent groups Compares the effects of different treatment conditions on preexisting groups of subjects.
• Longitudinal Investigates changes across time by measuring behavior of same group of subjects at different points in time.
• Cross-sectional Investigates changes across time by comparing groups of subjects already at different stages at a single point in time
• Pretest/posttest Explores the effects of an event (or treatment) by comparing behavior before and after the event (or treatment).
• In ex post facto studies participants would be separated into different groups according to their characteristics