I have a project where I have access to entire populations' records. I'm not trying to generalize knowledge to a larger population; rather, I'm trying to compare the prevalence of an event happening in one whole population to the next (think comparing what percent of shoppers buy a certain product at Walmart versus Target, where you have records on every single shopper at both).
But, to continue using this example, some stores are very small--maybe only 5 customers in a month, as opposed to 500 at another.
I want to figure out what my minimum population size should be to include in the comparison. I have to keep stores separate and not lump them together. I've tried to use sample size power estimations, but it doesn't seem like the right fit for the dilemma. But perhaps I'm wrong.