At most North American institutions the latter two verdicts are extremely rare, for two reasons. First, to obtain the status of doctoral candidates, graduate students typically pass a qualifying examination or comprehensive examination, which often includes an oral defense. Students who pass the qualifying examination are deemed capable of completing scholarly work independently and are allowed to proceed with working on a dissertation. Second, since the thesis supervisor (and the other members of the advisory committee) will normally have reviewed the thesis extensively before recommending the student to proceed to the defense, such an outcome would be regarded as a major failure not only on the part of the candidate but also by the candidate's supervisor (who should have recognized the substandard quality of the dissertation long before the defense was allowed to take place). It is also fairly rare for a thesis to be accepted without any revisions; the most common outcome of a defense is for the examiners to specify minor revisions (which the candidate typically completes in a few days or weeks).