Bailey (1973) and Bickerton (1971) were particularly critical of such attempts to use variable rules. They acknowledge variability in language but insist that it can be explained if we look closely at the environments in which variation occurs and are prepared to relate the environments to one another using some kind of scale. We must note, of course, that they are concerned with individual speech behavior, what they call the isolect, whereas Labov and others have been concerned with group behavior, the sociolect, insisting that such behavior is important in studies of how people actually use language not only to communicate verbally, but for a variety of other purposes too.

(from Wardhaugh, R. (1998) An Introduction to Sociolinguistics. Oxford: Blackwell.)