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I have conducted repeated measures ANOVA, with 2 factors: One within factor (time, pre-post intervention) and one between factor (treatment, two groups). I have measured different dependent variables. I find in all the cases an effect of the within factor, an effect of the between factor, and only in some cases an interaction between both. I want to know how to interpret these results.

  • What does the interaction mean?
  • Is the significance of the factor treatment (between) sufficient to be able to say that one group is superior to other one, that one treatment is better?

EDIT: The measures I took in three moments are: time spent using technologies, frecuency of use, abuse.. The significant interaction is for within factor (moment) X between factor (group), but it's only significant for the time spent using technologies. What means the sugnificance of between factor? Why it's important the intetaction? Thanks


The measures I took in three moments are: time spent using technologies, frecuency of use, abuse.. The significant interaction is for within factor (moment) X between factor (group), but it's only significant for the time spent using technologies. What means the sugnificance of between factor? Why it's important the intetaction? Thanks

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I think you need to be more specific about the problem. What is the model? What factors were significant and which ones weren't? –  Michael Chernick Aug 31 '12 at 20:06
    
If you have both pre vs. post & treatment1 vs. treatment2, & they were significant but the interaction wasn't, then you may have had a failure of randomization. You want your pre-treatment observations to be equivalent & only your post-treatment observations to differ. Ie, you want your interaction to be significant, but your main effects not to be. Also, if you have >1 DV, you should probably be doing MANOVA. –  gung Aug 31 '12 at 22:46
    
Then, the most important result is the interaction? The allocation to the different conditions was random. Thanks –  Clara Sep 1 '12 at 9:29
    
This is a program of prevention of technological addictions. One of the conditions or groups is the traditional program and the other contains additional control techniques. The total sample comprised 1160 students. I have three measures: pre, post and follow-up. –  Clara Sep 1 '12 at 9:41
    
The dependent variables are several: the frequency of use, time of use, the degree of dependence ... There are no differences between the previous groups, homogeneous groups. I've done ANOVA and MANOVA. For example, I find an effect of between and within factor when I take pre-post dates. And only appears a significant interaction (time X group) for the time of use. My doubt is what means each one of the effects. I don't know if I can say that by having a significant factor between, there is a treatment that produces a superior improvement. I don't understand the meaning of the interaction –  Clara Sep 1 '12 at 9:42

2 Answers 2

I assume time of use refers to an addiction (such as use of a drug). Also when you say dependent variables i think youprobably mean the predictors (usually called independent variables). Time of use interacting with treatment group could mean that the effect of the intervention can be stronger/weaker depending on how recently the subject was using the drug for example.

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The measures I took in three moments are: time spent using technologies, frecuency of use, abuse.. The significant interaction is for within factor (moment) X between factor (group), but it's only significant for the time spent using technologies. What means the sugnificance of between factor? Why it's important the intetaction? Thanks –  Clara Sep 2 '12 at 10:44
    
I think the meaning is exactly what I said. At the three different times the time spent using the technologies changes significantly. But it doesn't for the other variables (abuse and frequency of use). That is all there is to interpret. Regarding the importance of the interaction, it is important because it says that time spent using technologies will not have the same effect at different times. –  Michael Chernick Sep 2 '12 at 11:06
    
Thanks. And how can I know if one of the programs is better? Then, what means the significance of between groups factor? –  Clara Sep 2 '12 at 11:21

What does the interaction mean: It means the effect of treatment differed as a function of time. This is almost certainly the outcome you desire.

Is the significance of the factor treatment (between) sufficient to be to say that one group is superior to other one, that one treatment is better: Almost certainly not. As mentioned in the comments this might suggest that your groups were actually different at pre - intervention.

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