Some scholars have done studies attempting to establish super-families. They have, for example, linked Indo-European, Uralic, and other families into Nostratic. Such attempts have not been accepted widely. The information needed to establish relatedness becomes thinner the farther back. The time depth of linguistic methods is limited because of chance word resemblances and variations between language groups. Scholars can trace language changes back to around 10,000 years. The dating of the various proto-languages is also difficult; several methods are available for dating, but only approximate results are possible.