Participant Observation Has Been Used Widely in Research on a Range

Participant observation has been used widely in research on a range of religious groups, but it suffers from the following limitation(s) :

A) it is such a quick way to gather information that it tends to be overused.
B) observations may be biased by the observer's opinions and beliefs.
C) it provides quantitative data and that is a problem when studying religion.
D) it manipulates some variables and controls others; therefore is unethical when studying people's religion.