Day 5: Poisson Distribution I
Day 5: Poisson Distribution I
Terms you'll find helpful in completing today's challenge are outlined below.
Poisson Random Variables
We've already learned that we can break many problems down into terms of , , and and use the following formula for binomial random variables:
But what do we do when cannot be calculated using that formula? Enter the Poisson random variable.
Poisson Experiment
A Poisson experiment is a statistical experiment that has the following properties:
 The outcome of each trial is either success or failure.
 The average number of successes that occurs in a specified region is known.
 The probability that a success will occur is proportional to the size of the region.
 The probability that a success will occur in an extremely small region is virtually zero.
Poisson Distribution
A Poisson random variable is the number of successes that result from a Poisson experiment. The probability distribution of a Poisson random variable is called a Poisson distribution:
 is the average number of successes that occur in a specified region.
 is the actual number of successes that occur in a specified region.
 is the Poisson probability, which is the probability of getting exactly successes when the average number of successes is .
Example
Acme Realty company sells an average of homes per day. What is the probability that exactly homes will be sold tomorrow?
Here, and , so
Example
Suppose the average number of lions seen by tourists on a oneday safari is . What is the probability that tourists will see fewer than lions on the next oneday safari?
Special Case
Consider some Poisson random variable, . Let be the expectation of . Find the value of .
Let be the variance of . Recall that if a random variable has a Poisson distribution, then:

Now, we'll use the following property of expectation and variance for any random variable, :So, for any random variable having a Poisson distribution, the above result can be rewritten as: