I'm about to do a study about the merits of one stimulus compared to another with a within-subject design. I have a permutation scheme that is designed to reduce order effects of some parts of the study (task type order, stimulus order, task set order). The permutation scheme dictates that the sample size be divisible by 8.
To determine the sample size I'd have to either take a wild guess (being a good tradition in my field) or calculate the sample size for my desired power. The problem is now that I don't have the slightest clue how large an effect size I'm going to observe (also a good tradition in my field). That means power calculation is a bit difficult. Taking a wild guess, on the other hand, might also be bad because I can either come out with too low a sample size or pay too much money to my participants and spent too much time in the lab.
Is it okay to state upfront that I add participants in batches of 8 persons until I leave a corridor of two p-values? E.g. 0,05 < p < 0,30? Or how else, would you recommend, should I proceed?