Let's compare Poisson to binomial.
In a binomial problem, the number of trials (n) sets the limit on the values that X can
1) if the experiment is 6 questions that the student might guess on, and success event is
guessing them correctly, then the X could take on the values: 0, 1, 2, 3, 4, 5, 6 b/c she
might guess none correctly, or one , or two, or .
...., all the way to 6.
2) if the experiment is number of sales that a vendor makes when making 5 sales calls,
then X will take on values: 0, 1, 2, 3, 4, 5 b/c he might make no sales or all the way up to
5 sales (if he's having a good day).
So, in binomial variables, the number of trials "n" is the cap on the values that X can take
on. Also, remember from ch. 4: the summation of the probabilities for all the X values
must be 1.0. So, for all binomial variables the following holds: f(0) + f(1) + f(2) + .
f(n) = 1.0 for a binomial random variable.
Now, compare this with a Poisson variable. In this case, X is the number of occurrences