Ex Post Facto Designs: Definition & Examples

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  • 0:07 Ex Post Facto Defined
  • 2:56 Example
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor: Devin Kowalczyk

Devin has taught psychology and has a master's degree in clinical forensic psychology. He is working on his PhD.

This lesson explores what an ex post facto design is using two different examples. In addition, specific attention is paid to differentiating ex post facto from true experiment to reduce confusion.

Ex Post Facto Defined

Sometimes you want to study things you can't control - things you can't ethically or physically control. For instance, you can't make someone overweight to study the effects it has on their brain. You can't alter someone's eyesight to see how it affects their motor skills.

Ex post facto design is a quasi-experimental study examining how an independent variable, present prior to the study, affects a dependent variable. So like we just said, there is something about the participant that we're going to study that we don't have to alter in the participant. We will make this a little clearer a little later with some examples and descriptions.

But first, quasi-experimental simply means participants are not randomly assigned. In a true experiment, you have what is called random assignment, which is where a participant has an equal chance of being in the experimental or control group. Random assignment helps ensure that when you apply some kind of condition to the experimental and control groups, there isn't some predisposition in one group to respond differently than the other.

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