When is a person entitled to examine the scholars' evidence in matters of differing opinions, attempt to weigh them, and adopt what he deems most preponderant?

People are divided into three categories: a knowledgeable mujtahid (independent jurist), a discerning student of knowledge, and a common person. A discerning student of knowledge examines the evidence presented by scholars in matters of اختلاف (disagreement) and adopts what he deems most preponderant based on his understanding of the rules of language, fiqh (jurisprudence), and its usul (principles). If the preponderance relies on authenticating or weakening chains of narration, then it is essential to be familiar with the rules of hadith methodology.