We have two options for serving ads within Newsfeed: 1 – out of every 25 stories, one will be an ad 2 – every story has a 4% chance of being an ad For each option, what is the expected number of ads shown in 100 news stories? If we go with option 2, what is the chance a user will be shown only a single ad in 100 stories? What about no ads at all?

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  • The expected number of ads shown in 100 new stories for option 1 is equal to 4 (100/25 = 4).
  • Similarly, for option 2, the expected number of ads shown in 100 new stories is also equal to 4 (4/100 = 1/25 which suggests that one out of every 25 stories will be an ad, therefore in 100 new stories there will be 4 ads)
  • Therefore for each option, the total number of ads shown in 100 new stories is 4.
  • The second part of the question can be solved by using Binomial distribution. Binomial distribution takes three parameters:
  • The probability of success and failure, which in our case is 4%.
  • The total number of cases, which is 100 in our case.
  • The probability of the outcome, which is a chance that a user will be shown only a single ad in 100 stories
    p(single ad) = (0.96)^99*(0.04)^1