Speech community is heterogeneous, displays linguistic variation
Much of that variation is not random, but orderly displaying systematic correlations with social factors (such as age, sex, social class) and speech style (formal vs. informal)
An adequate theory of language must include a theory of linguistic variation, rather than dismissing variability as theoretically unimportant
Competence-performance distinction is artificial
Communicative competence: Performance - variation (who uses which variable forms in which social contexts) is one of the things that native speakers "know" about their language
Empirical competence: Linguistic theory should not be based on speakers' intuitions or analysis
Observer's paradox: How to observe language without observing language... Intrusive effect on linguistic behaviour
Vernacular: the way people talk when they are not being observed (unselfconscious speech)
Orderly heterogeneity: Systematic observation of vernacular speech