However, modern studies (1) have shown that many early authors—such as Ibn Saʿd, Khalifah ibn Khayyat, Muslim ibn al-Ḥajjāj, and others—did not treat the term ṭabaqa (stratum) as a fixed unit of time; nor did they use it to denote an entire "generation." Thus, while Khalifah ibn Khayyat regarded the Companions as a single stratum, Ibn Saʿd divided them into several strata on the basis of their precedence in embracing Islam.

As for the strata of the Successors and those who came after them, the scheme of Khalifah and Ibn Saʿd rests on whether or not there was actual contact between the Companions and the Successors. The senior Successors are those who transmitted from the senior Companions distinguished for their early conversion and merit; these constitute the first stratum of the Successors. Successors who transmitted only from the junior Companions and never met the senior ones—because they did not live long enough—are assigned to a third or fourth stratum. Likewise, anyone who narrated from Saʿīd ibn al-Musayyab, for instance, or from other leading Successors is placed in the first stratum of the Followers of the Successors.

Fourth – The notion of the ṭabaqa in al-Siyar and other works of al-Dhahabī
Imam al-Dhahabī arranged a number of his books—besides al-Siyar—according to strata, among them: Tadhkirat al-Ḥuffāẓ (2); Maʿrifat al-Qurrāʾ al-Kibār ʿalā al-Ṭabaqāt wa’l-Aʿṣār (3); al-Muʿīn fī Ṭabaqāt al-Muḥaddithīn (4); and al-Mujarrad fī Asmāʾ Rijāl Kitāb Sunan al-Imām Abī ʿAbd Allāh Ibn Mājah, apart from those among them for whom he recorded a narration in any