This almost answered my question below, but not quite.
I want to calculate the sample size required in order to reach a certain level of a priori statistical power in my experiment. My question is about what ‘sample size’ means in this type of calculation. Does it mean the number of participants or the number of data points? If there is one data point per participant, then these numbers will obviously be the same. However, I’m using a mixed-effects logistic regression model in which there are multiple data points nested within each participant. (Each participant produces multiple ‘yes/no’ responses.)
If the number of data points is what matters, and I am using GPower, do I just enter this number into the 'sample size' window, with no adjustments anywhere else in GPower?
It would seem odd if the calculation of a priori statistical power did not differentiate between whether each participant produces one response or multiple responses.