That's not a source, that's a number. You have to link the sources, you can't just paste Wikipedia. In any case this is a discussion about specifically the American middle class.

Edit': Also I found the Wikipedia article you're citing and it directly contradicts your point: "The modern usage of the term "middle-class", however, dates to the 1913 UK Registrar-General's report, in which the statistician T.H.C. Stevenson identified the middle class as those falling between the upper-class and the working-class.[14] The middle class includes: professionals, managers, and senior civil servants. The chief defining characteristic of membership in the middle-class is control of significant human capital while still being under the dominion of the elite upper class, who control much of the financial and legal capital in the world."