Study of the men. Refers to study of the people who transmitted hadith reports, manifested in the production of biographical dictionaries (tabaqat). Aimed to demonstrate their moral character as a means of validating the reliability of those reports. The details of the geographical and historical events in a person's life allowed a judgment to be made on the likelihood of a given transmitter having received a report from another transmitter. When this was combined with assessments of the transmitter's character, theological affiliation, and mental capacity, rankings of reliability were established. Chains of transmission (isnads) containing “weak links” were, in theory, eliminated. Assessments were codified starting in the tenth century in a specific genre often called al-iarh wa'l-tadil (rejecting and admitting).