Define probability mass function?

Question

I am in a Stastics intro class and would like a good, simple definition of probability mass function

Answer 1

mathematical definition; all proportions are ≥ 0, the sum of all proportions in the domain is 1.0 and the proportion that x takes on certain values say x1, x2,..xn is the sum p(x1) +p(x2) =…p(xn)

Example

x:   p(x)

0  .0001

1  .0036

2  .0486

3  .2916

4 .6561

total - 1.000

The table shows the proportion for the values 0,1,2,3,4. The function does not exist for any other values.  The proportion that is 2 or less is .0001 + .0036 + .0486. Be careful, for a mass function ≤ is not the same as <.