Why it is important to learn the difference between median and average

One of my favorite policy wonks is Matthew Yglesias, and I especially love his love of graphs.  There are stories that make you pause because they crystallize abstract issues or complex ideas, and this post of his does just that.  I have excerpted the graph, which he takes from Lane Kenworthy, because it perfectly encapsulates the greatest economic and political issue of the previous 30 years: 

Republicans prefer to talk about the average; Democrats, the median.

In a society with an income distribution in the shape of a bell curve, the median and average incomes would be the same.  In a country where the income is skewed to the wealthy, like America, the average income will be much higher than the median.  (The average of the sequence 1,2,3,4,100 is 22 but the median is only 3.)  The graph shows us that the previous 34 years have witnessed an expansion of wealth directed towards the already well-off members of society (and I guess that as their cups filled, the left over wealth trickled down, which is why median income also increased).  The graph also shows how economic conservatives and defenders of the status quo do not lie when they claim that income growth has continued at its historical rate.  They just neglect to mention that they are discussing average, not median, income.  If average growth had also flatlined along with the median’s, explanations focsing on globalization, returns to college education, and immigration would have merit.  Instead, the graph is one more piece of evidence damning the dominant ideologies of the previous 30 years.