Psychology Random Assignment

At the end of the experiment, the experimenter finds differences between the Experimental group and the Control group.Because each participant had an equal chance of being placed in any group, it is unlikely the differences could be attributable to some other preexisting attribute of the participant, e.g. Random assignment does not guarantee that the groups are matched or equivalent.

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At the end of the experiment, the experimenter finds differences between the Experimental group and the Control group, and claims these differences are a result of the experimental procedure.

However, they also may be due to some other preexisting attribute of the participants, e.g.

A study using only random selection could randomly select students from the overall population of the school, but then assign students in one grade to the intervention and students in another grade to the control group.

While the data collected from this sample could be used to make inferences about the population of the school, the lack of random assignment limits the conclusions that can be made about the intervention’s effect.

people who arrive early versus people who arrive late.

Imagine the experimenter instead uses a coin flip to randomly assign participants.

If the coin lands heads-up, the participant is assigned to the Experimental Group.

If the coin lands tails-up, the participant is assigned to the Control Group.

This is a rare event under random assignment, but it could happen, and when it does it might add some doubt to the causal agent in the experimental hypothesis.

Random sampling is a related, but distinct process.