Interpreting data

The Washington Post today published an article that showed the following map with average annual salary of teachers in public elementary and secondary schools.

Looks pretty bad, doesn't it. Most of the country in green seems to be paying too little to its public school teachers.

But look at this map of the average cost of living:

A lot of the states with low teacher salaries have low costs of living; many of the states with higher teacher salaries have higher costs of living. Thus in some states each dollar in the teacher's salary goes farther than it would in other states.

Now compare the two with this map of average income by county  in the USA.

Maybe the people in the north east and in California, with higher than average income, can afford to pay their teachers more than people in states with lower average incomes. Maybe they also need to do so to meet the higher costs of living in their states (if people get paid more, it tends to cost more to go to a restaurant, get a hair cut, or go to a doctor).