Fig 1.

Self assessment: how much difficulty do you have in moving around? The problems of interpersonal or cross population comparability may be conceptualised in terms of shifts in response category cut points. Different people (A, B, and C) might translate levels on an unobserved, continuous mobility scale into categorical responses in different ways, depending on the location of their cut points. Cut points define thresholds on the unobservable scale at which individuals move from one response category to another