Random Assignment of Ad Impressions Across Users
Setup
In an A/B experiment, Y ad impressions are served uniformly at random across X distinct users. Each impression is independently assigned to one of the X users; multiple impressions can be delivered to the same user.
Questions
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For an arbitrary user i, what is the expected number of impressions they receive?
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What is the probability that user i receives at least one impression?
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Provide both the exact expressions and the large-scale Poisson approximations.