> - 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.
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.