Overt Participant Observation Previous Page Naturalism
The problem here is that, even though people are being studied in their natural environment / habitat and the research is carried-out with the co-operation of the people involved, there is no way of measuring how the presence of the observer influences the behaviour of group members. Just act normally...

The basic question, therefore, 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 observer's presence produce an unknown level of change in people's behaviour? W.F.Whyte ("Street Corner Society") recognised - but never really solved - this problem.