I am evaluating the effect of an intervention (treatment: a lecture) on the ability to spot behavioral clues of emotions using visual stimuli using pretest-posttest plus control group experimental design.
The experimental group is given the first set of stimuli, then there is intervention and the second set of stimuli follows. Naturally, the same without the intervention occurs in the case of the control group. Two groups, two measurements each.
1. How to determine the number of subjects needed in each group? (Ideally using G*Power 3 – my problem is that I literally do not understand which method should I pick [I guess ANOVA: Repeated measures, between factors] and on what basis are the desired power and effect size determined.)
Suppose that I have got data from 30 experimental and 20 control subjects. The most appropriate method to use seems to be the rANOVA (GLM -> Repeated Measures in SPSS).
Data example:
group pre post
1 exp 10 15
2 exp 8 9
3 con 8 7
4 con 5 6
2. How to perform the same analysis in R?
3. After the analysis, which data are best to describe the significance of the intervention?
Apparently, I am a total newbie and indeed a confused one because for the last two days, I have been searching and found many contradictory claims, suggesting two paired t-tests or Mann-Whitney U, without discussing the procedure of choosing appropriately sized groups in a practical way.
Thanks for any help, improving suggestions are welcome.
