al-Taqrīb (1).

Hadith scholars devised the stratification system to serve the study of the noble Prophetic hadith, enabling them to identify and critique each chain of transmission. By means of this system one can tell whether a sanad is continuous, or whether it contains an irsāl (2), an inqiṭāʿ (3), an ʿaḍal (4), a case of tadlīs (5), or an instance where narrators share the same name while belonging to different strata (6).

In the earliest centuries—when authors paid little attention to recording narrators’ birth and death dates—the stratification method was indispensable, for a narrator’s stratum could be established only by knowing his teachers and his students.

One of the major drawbacks of arranging material by strata, however, is that anyone not thoroughly trained in the discipline finds it difficult to locate a particular biography, quite apart from the absence of a uniform definition of strata among the authors themselves.

Once sufficient data on births and deaths became available, more and more scholars began to organise their biographical works either by year of death or by alphabetical order.

Among the criticisms Abū al-Ḥajjāj al-Mizzī levelled at Ḥāfiẓ ʿAbd al-Ghanī al-Maqdisī regarding the arrangement of his book al-Kamāl fī Asmā’ al-Rijāl was that he separated the Companions’ biographies from the rest. Al-Mizzī said, “The author—may Allah have mercy on him—began by listing the male and female Companions separately, and then mentioned those who came after them separately; we therefore considered it preferable to combine everyone…"