Greenberg's Eurasiatic hypothesis has been dismissed by many linguists, often on the ground that his research on mass comparison is unreliable. The primary criticism of comparative methods is that cognates are assumed to have a common origin on the basis of similar sounds and word meanings. It is generally assumed that semantic and phonetic corruption destroys any trace of original sound and meaning within 5,000 to 9,000 years making the application of comparative methods to ancient superfamilies highly questionable. Additionally, apparent cognates can arise by chance or from loan words. Without the existence of statistical estimates of chance collisions, conclusions based on comparison alone are thus viewed as doubtful.