# Comparing means between two groups over three time points

I have two groups of ten observations each, where the weight are measured at three different points in time. Let´s say baseline, 2 months and 5 months after baseline.

Now I would like to draw a conclusion if the mean of one group is lower than the other group over time. They most likely weigh the same at the first measurement, with little variation.

There´s been a suggestion to fit a simple linear regression, where we use the difference in mean of the two groups as response and time as dependent. In my opinion it seems wrong, we will only have three observations. Would it be more correct to fit som kind of Repeated measurements ANOVA or any other suggestion? Thankful for any input.

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Your's is a classic example for RM-ANOVA. You have 3 effects to check in it: (1) Between-groups difference combining the 3 times, (2) Within-subject difference, i.e. whether there is time effect combining the 2 groups, (3) Interaction between-within effect, i.e. whether 2 groups react differently through time. One side notion: if you deal with body weight, it is likely to be right skewed, so it's worth considering a transformation first, perhaps. –  ttnphns Jul 3 '12 at 6:56
Thank you for your response. I have an additional thought/question about the randomization. Only the obersvations/subjects has been randomly selected, could this be a problem when evaluating the model? for example time here cant be randomized and which group observations belongs isnt either randomized. Dont know if it makes sense, thankful for any response. –  Carlos Aug 7 '12 at 15:13