Sample size (or number of clusters) by treatment arms
Pilot: 145 subjects, randomizing 3x3, i.e. 15 in each subgroup
The following are estimates, since I will base the number of tasks on the results of the pilot, which will give an idea over the statistical power I will have and (given my fixed budget) allow me to compute the sample size.
Between subjects design: 100 in total, 50 in each subgroup (high effort first vs low effort first)
Within subjects design: 200 in total, 100 per subgroup (high effort first vs low effort first)
Repetition a few months later: Randomly chosen half from the 'within-subjects' group - that is, another 100