Personal probabilities: A radically special approach for allocating probabilities to events than, for instance, the commonly used long-term relative frequency approach. In this type of approach, probability represents the degree of belief in a proposition, based on the entire information.

The two people with different information and different subjective ignorance might therefore assign different probabilities to the same proposition. The only constraint is that the single person's probabilities must not be inconsistent.
Person-years at risk Units of measurement which combine the persons and time by summing individual units of the time (the years and fractions of years) during which subjects in a study population have been exposed to the threat of the result under study. A person-year is equivalent of the experience of one individual for one year.