Can someone provide a valid definition or explanation of the concept "significant main effect"?...?

Question

I am not a lazy student or an inept practitioner, but I have to read a lot of social science literature and keep running across this phrase. I am long out of school and this didn't come up in my required psych course. I have not been able to find a clear explanation of how a "significant main effect" is established or supported by data. From context I have begun to get the feeling that researchers report a significant main effect when none of the tested hypothesis are supported ... ( I know that sounds cynical - but it is just an impression) and I still don't know where is comes from statistically or in relation to the study design. Can anyone explain, especially anyone with some credentials or a cited source?

Amy · Accepted Answer

Generally, it means that an independent variable in an experiment was manipulated in such a way as to cause a change in the dependent variable, such that the result is statistically significant.

For example, if you give different groups of subjects different amounts of alcohol to drink (making sure you control for differences in body weight, gender, etc.), then test their reaction times (like, how quickly they can press a button when the letter "W" pops up on a screen), your independent variable is alcohol consumption, and your dependent variable is reaction time.  If the reaction times for each group of subjects are slower according to amount of alcohol consumed, then amount of alcohol has a significant main effect on reaction time.  BY ITSELF, alcohol increases reaction time.

(Not that you'd probably ever get to conduct this experiment in most places, due to ethical considerations.  Darn it.)

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Can someone provide a valid definition or explanation of the concept "significant main effect"?...?

1 Answer