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.

A true experiment and ex post facto both are attempting to say: this independent variable is causing changes in a dependent variable. This is the basis of any experiment - one variable is hypothesized to be influencing another. This is done by having an experimental group and a control group. So if you're testing a new type of medication, the experimental group gets the new medication, while the control group gets the old medication. This allows you to test the efficacy of the new medication.

Ex post facto designs are different from true experiments because ex post facto designs do not use random assignment. True experiments have random assignment because you're looking at something else. In ex post facto, you are looking at a prior variable present in the participant.

In an ex post facto design, you are not randomly assigning people to an experimental group or control group. You are purposefully putting people in a particular group based on some prior thing they have. I say 'thing' because it could be 'must have glasses,' or 'must be overweight.' There is no limit to the ways you could divide up the population.

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