Biographical evaluation (`ilm ad-diraya, `ilm ar-rijal) exists both in Sunni and Shi`i branches, and it refers to the strengthening and weakening of individual narrators & transmitters, and chains of transmission (isnad, or plural: asaneed). The purpose of the system is to grade hadith reports based on the trustworthiness of its transmitters. To summarize the Sunni system, all companions of the Prophet (pbuh) - ie all of those who have been in his presence at some point in his life - are considered trustworthy (thiqa). These companions then narrated their traditions to their pupils, family members, and associates. They would then pass it down until they reached a compiler of hadiths, usually in oral form, but sometimes written.
The Sunni system excels in its biographical documentation because it covers a vast amount of individuals, giving relevant data about many people. But the system does have its flaws:
1) We don't consider all companions to be trustworthy; and we particularly distrust those who have directly oppressed the Prophet's family.
2) The culture of memorizing, transmitting, and documenting hadiths did not receive widespread popularity until the 2nd century AH. Therefore, the careful preservation of these hadiths are in question. Sunni isnads tend to be long, transmitted orally over centuries.
3) Strengthening (tawtheeq) is based mainly on scholarly opinion, with much disagreement.
Shi`i hadiths take a different approach. The vast majority of Shi`i hadiths come from one of the twelve Imams. The Shi`a hold the belief of a golden chain, which is the chain from one of the Imams that goes through his forefathers back to the Prophet (pbuh). Through the hadith of thaqalayn, the Prophet established that the Qur'an and Ahl al-Bayt are what the Muslims must hold onto, and that the two are one in essence. The Ahl al-Bayt are (at least primarily) the 12 Imams + Fatima (as). In many hadiths, the Prophet aligned himself with `Ali and Fatima, saying the truth is with them, that whoever angers them angers the Prophet, that opposing them is hypocrisy and disbelief, etc. The tying of truth with `Ali, the Mahdi, etc. gives them high authoritative value. The Imams have said in many hadiths that all they say and do comes from the Prophet. Many times, they quoted the Prophet directly, and they have said that all quotations of the Prophet come from their golden chain to him. Likewise, as infallible guides, all that they say and do is from the Qur'an and Sunna, and therefore their words are taken as proof (hujja) for all religious matters.
This means that the relation of hadiths in Shiism took place over a 300+ year period rather than just a 23 year period. Surely, the religion was completed and perfected by the end of the holy Prophet's lifetime. That same religion was relayed by the Imams. As hadith narration became popular in the second century AH, thousands of students studied under the 5th and 6th Imams. Together, al-Baqir and as-Sadiq narrated tens of thousands of hadiths on all topics - `aqeeda, fiqh, tafsir, history, eschatology, and more. The Imams gave their students the explicit instruction to write their words down, memorize their hadiths, and spread the knowledge to the people. Hence, the hadith collection process began in their lifetimes. The earliest available Shi`i notebook (usl) dates back to the time of the 4th Imam. By the occultation of the 12th Imam, over 300 of such usool existed. Unlike Sunni tradition, the hadiths were mostly not transmitted orally between the Prophet and a third century compiler. Rather, the hadiths came mainly from the Imams, and most of them were copied down during the time of the Imams. In some books, the chains of narrators are considerably shorter than in Sunni books. The time between the narration of the hadith and its compilation is also much smaller.
As noted earlier, not all companions of the Prophet - or the Imams - are considered reliable. Their veracity and loyalty to Ahl al-Bayt must be proven. There are many ways that a hadith narrator is given tawtheeq:
1. The Imams directly gave tawtheeq to some people.
2. The Imams gave taraddi (expressing God's satisfaction) and tarahhum (asking God's mercy) to some people.
3. Like in Sunni rijal, the scholars would give tawtheeq to people or weaken them, based on their biographical data, beliefs, actions, who they associate with, etc.
4. The clients, messengers, and tax-collectors of the Imams were largely given tawtheeq.
5. People can be given tawtheeq through other thiqa people.
6. People can be given tawtheeq if they are relied upon by major trustworthy companions of the Imams (as`hab al-ijma`)
And many other means.
There are certain levels that a narrator can embody.
1. A narrator can be considered thiqa. This means the narrator is trustworthy in what he narrates. Non-Shi`is can be considered thiqa, but this will be noted in the grading of the chain. A sahih chain is one where all the rijal are Imami Shi`a. A muwathaq chain is a chain that is all thiqa, but may include trustworthy Sunnis, Zaydis, Fat`his, Waqifis, etc.
2. A narrator can be considered `aadil or faadil or mamdooh which means that he is a just and good person, but his explicit tawtheeq cannot be established. This makes a chain hasan in grading.
3. A narrator can be considered dha`eef, which means he is weak. Either he is known for lying and bad character, or he is associated with the enemies of Ahl al-Bayt (nawasib, or ghulat - Shi`i extremists), or both.
4. A narrator can be considered majhool, which means we may know some biographical details about the person, but not enough to establish trustworthiness or lack thereof.
There is a theory called as`hab al-ijma` that is used by a minority of scholars. The as`hab al-ijma` are a list of 18 companions of five of the Imams who are considered very trustworthy central figures of the sect. This method says: any hadith that is authentic up to one of these 18 can be accepted. Even if one of these 18 individuals narrate from someone without tawtheeq, the idea is that they would not relate a hadith unless it had value - as they were close, accepted, and tested supporters of the Imams. However, to be safe and cautious, many rijal scholars do not use this method.
The hadiths parimarily came from the Imams during their time in Medina. Their Shi`i partisans were mainly Kufan visitors who would go to Medina, stay for a while, gather knowledge and bring it back to Kufa. As mentioned before, Kufa and Baghdad were an Islamic powerhouse during the second century AH, and most of what was written in the early period in both sects was in Iraq and Persia. That is where most Muslim scholars came from and most Islamic books were written. Thus, the tradition survives through this transmission. From Kufa, the hadiths also went to Qum when Ibrahim b. Hashim and others took their traditions there. There were thousands of Shi`as in Iraq during the time of the 6th Imam, and many hundreds of his companions were Kufan transmitters of hadiths.
A hadith or concept that is narrated through multiple chains is mutawater (widely narrated). `Aqeeda must be established on mutawater traditions. Fiqh however can be established throug ahad (single-authority) traditions.
There are some issues with rijal. We should recognize that it is still a man-made system and will have its faults. The main fault in Shi`i rijal is that there are too many majhool narrators, because the Ahl al-Bayt had thousands of students, and the status of many of them was not known to the scholars of rijal. Also, different scholars had different opinions on certain narrators. There are also some manuscript discrepancies in the works of some rijal scholars (most prominently, Ibn al-Ghada'iri's). Sometimes we don't have as many biographical details as we want. Rijal scholars largely lived after the people they had written about were dead. However, the system can weed out contradictions and strengthen established concepts. It is also an insurance that what we believe and practice was what the best of the Muslims believed and practiced.
The gradings of narrators are usually extrapolated from the biographical information provided by major Shi`i classical scholars of rijal. These scholars include Najashi (~ d. 1058), whose gradings are usually preferred, Ibn al-Ghada'iri (11th century), Shaykh al-Tusi (d. 1067), and Kashhi (d. 951). It is recorded that Shaykh al-Kulayni, the compiler of al-Kafi, and Shaykh as-Saduq had their own books of rijal, but those book have not survived. Furthermore, some scholars have accepted all of the narrators who have been included in Tafsir al-Qummi and Kamil az-Ziyarat, under the belief that the authors of these works have only included reliable narrators. Later scholars who have contributed to the science include `Allamah al-Hilli (d. 14th century), `Allamah al-Majlisi (d. 17th century), Shaykh Bahbudi (d. 20th century), Sayyid Burujirdi (d. 20th century), al-Khoei (d. 20th century), Muhammad Taqi al-Tustari (d. 20th century) Shaykh Asif Muhsini, Shaykh ar-Radi, Shaykh as-Sanad, and many others.
It should be noted that the authors of the Four Books - Kulayni, Saduq, and Tusi - took rijal seriously. They believed that their books were filtered enough to represent Twelver Shiism, even for lay use. Kulayni in particular viewed his work as sahih in content. Many attested to the works of these scholars and others. While some later scholars have weakened many narrations in the Four Books based on a strict adherence to classical rijal standards, this standard is seen by some scholars to be too stringent and unnecessary. Still, the study of rijal provides a wealth of information on our sources, and it remains a critical tool for scholars and seminarians.
That is some [very] basic information on rijal in Shiism - inshaAllah it is helpful to some.