I am using g*power to calculate the a priori power for a 2 group repeated measures anova with a continuous dependent variable with 3 time points. I think that higher values for the "correlation among rep measures" should indicate a better test (i.e., high test-retest correltaion) and thus yield better power and a lower total sample size; however the opposite seems to be true:
Test family: F tests; Statistical test: ANOVA: repeated measures, between factors
Effect size f: 0.25 alpha error probability: 0.05 Power: 0.8 Number of groups: 2 Number of measurements: 3
Correlation among rep measures: 0.8 (a reasonable value for these neuropsychological assessments of cognitive function) -> total sample size: 112
Correlation among rep measures: 0.5 (a poor test would have a value this low) -> 86
What am I not understanding? Thanks!