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I have a MINITAB printout I was hoping someone could help me with. My question is- from the table below how do I distinguish between a main effect and an interaction?

Study: Three drug treatments are compared at three sites. The treatments consist of two dosages (low and high dose) of an experimental drug and a control drug. The results were analysed by two-way ANOVA with interaction. The ANOVA is shown below (formatting improved).

Analysis of Variance for response

Source       DF         SS         MS       F      P
site          2     21.301     10.651    0.47  0.656
dose          2    185.404     92.702    4.09  0.108
site*dose     4     90.654     22.663    7.05  0.000    
Error        63    202.574      3.215
Total        71    499.933 
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  • $\begingroup$ This looks like homework... if you have a book or class notes it should be fairly clear from their definitions and examples of main effects and interaction effects which is which in this particular case. If you lack references just search for "anova interaction effect" $\endgroup$ – Michael Bishop Dec 4 '11 at 0:01
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site*dose is the interaction effect, site and dose are the main effects. Since the interaction effect is significant at a 5% level (p-value < 0.05). Thus, you need to test the simple effects in the next step.

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