The Mills ratio plays an important role in the famous "Heckman two-step" estimator that corrects for selection bias in the context of the bivariate normal selection model.
Each year when I lecture on this in my graduate applied econometrics course I try to remember whether it is the "Mill's ratio" or the "Mills ratio".
Wolfram and wikipedia agree that it is Mills, named after John Mills, and not "Mill's".
Left open is the question of whether to write "Mills ratio" as is common, or "Mills'" ratio, which comports with the usual rules of grammar.
I've updated my lecture notes and problem set and taken the easy way out by using "Mills" with no possessive apostrophe.
Hat tip: Dan Marcin, a student in this year's class
Addendum: Reader Nic Duquette suggests that, particularly in this case, the Chicago Manual of Style should be the guide.
Who was my favorite student this term?
10 months ago