I have a dataset that includes one IV with 3 levels (group 1, group 2, and group 3) and a repeated DV (pre test score and post test score). I am using R, but I'm unsure what codes I should use to test for the mean differences. In my dataset, each row represents a subject and so they are grouped by including a numerical value to each group 1 = First Group, 2 = Second Group, and 3 = Third Group.
I need to compare the mean decrease/changes over the 2 time periods(pre & post) across the groups to assess if there is a significant change in any of the groups. I think that ANOVA mixed-design/two-way repeated measures anova/split-plot anova is the right statistical technique. Can anyone help with the right code/package for this research-design? Also, how can I plot the outputs?