In my conference paper “Argument quality in Pulitzer Prize-winning reporting,” I say the following under the heading of “Problematic descriptive assumptions”:
If readers accepted the descriptive assumption that, usually, pay for police chiefs (or any city official) should be positively correlated with a city’s population or square mileage, then the comparison gave them reason to think that Adams’s salary was overkill.
But of course, that is a prescriptive, not descriptive, assumption.
I think I know what I was trying to get at by calling the assumption descriptive. The descriptive formulation of the assumption would be something like “The best way to shape a salary is….”
But the “should” is probably more accurate. It was silly of me to not recognize my conflation of descriptive and prescriptive assumptions. I should have written the heading more generically, like “Problematic assumptions.”