&gt; The definition of &quot;middle income&quot; is flexible.<p>The definition used is consistent: an household income of 2&#x2F;3 to double to the national median income.<p>The range shifts because the median income (even in inflation-adjusted terms dollars) shifts over time.<p>Aside from the complaints I&#x27;ve made about conflation of class and income elsewhere in this thread, the Pew definition is a reasonable and consistent way of defining a middle income tier over time, in a way which makes sense to talk about changes in size of that tier as an indicator of polarization of the income distribution, and which is a bit more intuitively understandable than some other distributional measures that relate to the same broad distributional concepts (e.g., the Gini coefficient of income.)