We have all been there: you want to fill in your pre-data collection registration for your new study and you need to input a number for the targeted sample size. Plus, you need to specify how you have come to that number. You turn to your favourite software (most likely G*Power) and, unless the design of the study is identical to the ones that you have used before, one thousand questions come to your head.
What does "number of groups" mean? The number of actual groups of participants? The number of levels in my variables? Also, what does "number of measurements" mean? The number of variables? The levels in them? Should I sum the levels of all variables? Should I multiply them? And, what is even worse, which measure of effect size should I use? D, eta, eta squared, F, the other weird greek letter...Should I leave the one by default in the software? I guess that must be the correct one, but then... How do I decide how much is a medium effect size?
If you want to dive deep and learn about all of that, you can go here, here. But if you only want to know which numbers to use, rest assure, I got you covered. You just need to know the size of the effect you would care about and which one is your key analysis. You can use the table below as an orientation on effect sizesÂ
Cohen's d (definition and calculation)
Cohen's f (definition and calculation)