This discussion was held on the underlying Planet Money article [1] [2]

The problem then, and the problem now, is that this is arbitrarily defining middle class. Can't we just call this a boxplot[3] of income in major cities, and not make this into a discussion of what "middle class means"?

[1] https://news.ycombinator.com/item?id=9237345 [2] http://www.npr.org/blogs/money/2015/03/19/394057221/how-much... [3] https://en.wikipedia.org/wiki/Box_plot