Is it a necessary consequence of a scholar's leniency in criticism (jarh) and disparagement (naqd) that he will also be lenient in authentication (tasheeh) and weakening (tad'eef)? And can a student of knowledge who is not specialized in Hadith exert his own effort (ijtihad) in authentication and weakening? What is the required methodology for that?

The leniency of some scholars in al-Jarh wa at-Ta'dil (discrediting and accrediting narrators) might be one reason for their leniency in authenticating Ahadith, but there isn't always a necessary correlation between the two. For example, Al-Hakim's leniency in "Al-Mustadrak" was not due to his leniency in al-Jarh wa at-Ta'dil, but rather due to his advanced age and weak memory, and his eagerness to complete his works. This led him into errors in his judgments, not in his narration, which was among his meticulously preserved principles. The scholars' mention of Al-Hakim's leniency pertains only to "Al-Mustadrak" and does not invalidate his other books on al-Jarh wa at-Ta'dil. It is impermissible to engage in Ijtihad (definition) (independent reasoning) concerning the authentication or weakening of Ahadith except for specialists. Scholars of al-Jarh wa at-Ta'dil must possess full expertise in Hadith (definition), its defects, and its narrators.