Theological Casuistry

Re: Ott (and similar manuals) weighing or attempting to weigh the certainty or probability of theological opinions -- upon what crtieria is one opinion deemed to be merely probable versus being of common consensus? (It's been a while since I've picked Ott up so I probably have the exact terms wrong.) Does the author look not just across space but across time? Is there any evaluation of the strength of the opinions themselves other than authority or number of adherents?