Interview Query

Ad Raters

Have you seen this question before?

Let’s say we use people to rate ads.

There are two types of raters. Random and independent from our point of view:

  • 80% of raters are careful and they rate an ad as good (60% chance) or bad (40% chance). 
  • 20% of raters are lazy and they rate every ad as good (100% chance).

1. Suppose we have 100 raters each rating one ad independently. What’s the expected number of good ads?

2. Now suppose we have 1 rater rating 100 ads. What’s the expected number of good ads?

3. Suppose we have 1 ad, rated as bad. What’s the probability the rater was lazy?

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