I have problem creating formula referencing a drop down list, can someone help?

Example:

In the drop down list, there a

1-Below Average
2-Average
3-Good
4-Excellent

1 is given 1 point, 2 given 2 points, 3 given 3 points and 4 given 4 points.
Assume there are 3 drop down list, all drop down list has 3-Good.
So the mean should be (3+3+3)/3 = 3

How do I create a formula that will calculate the mean?

Thanks for your help.