Holes, kinks and corners

As Pudney (1989) has observed, microdata exhibit “holes, kinks and corners.” The holes correspond to nonparticipation in the activity of interest, kinks correspond to the switching behavior, and corners to the incidence of nonconsumption or nonparticipation at specific points of time.

This is from “Microeconometrics”, by Colin Cameron and Pravin Trivedi and refers here.