> - is it a common problem to have, not knowing the set size? One motivating case is when the items arrive over time, like you have a website and you want to s

> - is it a common problem to have, not knowing the set size? One motivating case is when the items arrive over time, like you have a website and you want to sample uniformly from your visitors over the course of the next hour. You don't need to know how many people will arrive.