Effect sizes and repeated measures

#1
Hi there,

I am doing a bit of research looking at the effects of repeat test taking. Students are allowed to take a test multiple times. I have a dataset including student scores on each iteration of the test. I have done a repeated measures ANOVA to try to determine whether scores improve from one test iteration to the next.

According to the repeated measures ANOVA, scores do improve significantly with each iteration of the test. T1 = 26.99 (SD= 4.4), T2 = 29.12 (SD = 4.5). N for both groups is 2498. I calculated Cohen's D using an online calculator to give me an effect size of 0.48 (26.99-29.12/pooled SD).

Looking at the population of students who took the test 3 times: T1= 25.69 (SD= 4.2), T2 = 27.05 (SD= 3.9), T3 = 28.80 (SD=4.2). N for all 3 groups is 799. I used the online calculator to compute effect size for T1 to T3 = 0.77.

Just wondering if its valid to calculate an effect size from T1 to T3 or is this a no-no for repeated measures? I know that time 2 is not in the equation but is that ok if I explain the effect size is measuring the effect from T1 to T3?

Is there a better effect size for repeated measures? I also have figures for those who took it a 4th time (effect size is even bigger for that since the scores improve each time from T1 to T2 to T3 to T4).

Any advice appreciated.
Thanks.