Average payout calculation for variable payments: total cost of the session/# of subjects in that experiment session = $ per person.
For example, for a 60 min. experiment with a variable payment type, some individuals may receive less than $15 and some may receive more, but the average payment for that experiment should be at least $15/person.
The individual(s) who receive less than average must not be compensated by less than $5, which can be thought of as a minimum “show-up fee”.