Labov's Fourth Floor study (Language and occupation, accent and dialect, investigations)

This, in Labov's own words, is a useful explanation of the study. It shows how he took factors such as comparablity and the observer's paradox into account and it gives you more depth to use in your discussion. Try and read at least pages 168-170 inclusive but if you can bear to go further, do! Skim and scan the rest if you like and pick out what you can. Make good notes of years, findings, quotes, page references etc.