Full Idea
If a predicate G has a sharply-bounded set of cases falling in between the positive and negative, this shows that merely having borderline cases is not sufficient for vagueness.
Gist of Idea
If there is a precise borderline area, that is not a case of vagueness
Source
R Keefe / P Smith (Intro: Theories of Vagueness [1997], §1)
Book Reference
'Vagueness: a Reader', ed/tr. Keefe,R /Smith,P [MIT 1999], p.15
A Reaction
Thus you might have 'pass', 'fail' and 'take the test again'. But there seem to be two cases in the border area: will decide later, and decision seems impossible. And the sharp boundaries may be quite arbitrary.