Note Previous Page Observer Effect
The basic problem here is that of the extent to which people who know they are being studied change the way they "normally" behave. Is the observer seeing "normal behaviour" or does the mere fact the observer is present produce an"Yep. I always watch TV like this". unknown level of change (one that cannot be measured or quantified) in people's behaviour?

W.F.Whyte ("Street Corner Society") recognised - but never really solved - this problem. In a classic observation, the leader of the juvenile gang, "Doc", put his finger on this problem when he said:

"You've slowed me up plenty, now when I do something I have to think 'what would Bill Whyte want to know about it?'. Before I used to do things by instinct.".