The Addition Rule

In the previous video, we developed a multiplication rule for the probability that events A AND B both happen. In this video, we’ll consider how to calculate the probability that either event A OR event B (or both) happens.

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Notes on the video: The Addition Rule

A point to consider for this video:

In the birthweight example in this video, we considered the probability that a newborn baby boy was small (event A: birthweight ≤ 1600 g) or large (event B: birthweight ≥ 3750 g). Since a baby can’t be both small and large, P(A and B)=0. Events such as A and B, which cannot happen at the same time (so that P(A and B)=0), are called mutually exclusive or disjoint events.

  • How does the addition rule for two events simplify if the events are mutually exclusive?
  • Is it possible for two events to be both mutually exclusive and independent? What if the probability of one of the events is 0? What if the probability of neither of the events is 0?