Disability is not a Yes/No dichotomy but describes a dynamic and complex relationship between a person, the environment in which they live and their ability to participate in society on an equal basis with others. The aspects of functioning that are captured by the six questions are also not dichotomies; functioning in each of these domains exists on a continuum.
A question with a Yes/No response option forces the person answering to self-identify as having the difficulty or not. It is better to offer a range of responses as in the WG questions. The four answer categories recommended by the WG describe a continuum of functioning from: no difficulty to cannot do at all. The continuum can be visualized like this:
The distribution above creates four points equally distributed among the continuum that allows respondents to easily discriminate among options. The recommended cut-off (at a lot of difficulty) identifies those with the intended level of difficulties.