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.