Based in median income. For people that do not know what median income is:
If we have 5 people with the following income:
1: 10,000
2: 10,000
3: 25:000
4: 35.000
5: 1,000,000
The average income would be 216,000. This is an inaccurate representation. The median income would be based on the middle, which is 25,000, representing the population better (except person 5).