Universities' Collaboration in eLearning (UCEL)
A researcher observing a group of people through an observation panel without the group's knowledge may well be non-participant. Similarly, a researcher studying a situation in which they are taking a full part (e.g. in action research) will be a participant observer.
In most cases however, the researcher will be participating to some extent. Simply being present in a situation may affect what happens. Decisions about the extent to which the observer will participate should be made during the study design phase.
For example, if you will be a participant observer, will you continue to participate if asked to undertake dangerous or illegal practice? If you are to be non-participant, will you stand by if you observe dangerous practice, or identify a patient's need that is not being met by the team you are observing? In both of these examples there are ethical as well as practical issues to be considered.