text: The comparative method, a process of deduction, begins from a set of characteristics, or characters, found in the attested languages. If the entire set can be accounted for by descent from the proto-language, which must contain the proto-forms of them all, the tree, or phylogeny, is regarded as a complete explanation and by Occam's razor, is given credibility. More recently, such a tree has been termed "perfect" and the characters labelled "compatible".
  No trees but the smallest branches are ever found to be perfect, in part because languages also evolve through horizontal transfer with their neighbours. Typically, credibility is given to the hypotheses of highest compatibility. The differences in compatibility must be explained by various applications of the wave model. The level of completeness of the reconstruction achieved varies, depending on how complete the evidence is from the descendant languages and on the formulation of the characters by the linguists working on it. Not all characters are suitable for the comparative method. For example, lexical items that are loans from a different language do not reflect the phylogeny to be tested, and, if used, will detract from the compatibility. Getting the right dataset for the comparative method is a major task in historical linguistics.
  Some universally accepted proto-languages are Proto-Afroasiatic,  Proto-Indo-European, Proto-Uralic, and Proto-Dravidian.