Or, even better, if you're sampling from a data set as it's being generated. For example, let's say you want to grab 1,000 random tweets out of the output of Tw

Or, even better, if you're sampling from a data set as it's being generated. For example, let's say you want to grab 1,000 random tweets out of the output of Twitter's firehose for the next 2 hours - when you decide whether to keep tweet #1,329, you have no idea how many tweets are going to come in after that one, because they haven't been written yet!