Hi, I was just wondering if you could explain, in full detail what exactly the interaction effect entails.

Thanks! Melanie

-- Melanie N (melanien@yorku.ca), October 26, 2003

Answers

The simple effects for factor A are different at different levels of factor B. For instance, men and women are both given a drug that is supposed to raise their intelligence. The drug is found to raise the intelligence of women but not of men. So factor A is "Drug" which has two levels (control/placebo and drug) and factor B is "Sex" which has two levels (female and male), and IQ is the DV. There was a signficant effect for Drug (A) at the "female" level of Sex (B1), but not at the "male" level of Sex (B2).

There may or may not be significant main effects for Drug and for Sex (depending on how large the effect of the drug on women's IQ was), but the real "story" here is the interaction between Drug and Sex: women given the drug had higher IQs but everyone else's IQs were lower and (statistically) identical.

-- Christopher Green (christo@yorku.ca), October 26, 2003.