I'm designing pretty much a textbook instrumental variable study. Paraphrasing the actually set-up, we're planning to offer in-store discount coupons and trying to assess whether providing coupons is a good idea.
Given there will be a clear self selection involved (more loyal customers will opt in to more discounts) we're sending a mailer with discount to a random subset of customers and a mailer with no discount to the control. Assuming that we have a strong first stage, I'll estimate the true effect size of incremental discount via a 2SLS.
However, in this scenario, how can I decide how many people to send the mailers to so that I have a sufficiently powered experiment?