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Netflix has algorithms that decide how quickly your next dvd is sent. More accurately, it uses a score to rank the group of people who have the same DVD at the top of their queue in order of who should be allocated the next available copies.

This score is based on a number of factors, with the objective being to maximize subscriber happiness (one example question is whether is it more valuable to have quick turnaround for a frequent subscriber who is already happy with the service, versus an infrequent user who might be on the fence--it is not obvious who would be more sensitive to and negatively impacted by a delay in receiving the next DVD).

(Source: I work with people who previously worked at Netflix. This is not confirmed fact but a loose understanding based on casual conversations.)