Evaluation of Inquiry tools; Observation

Observation is one of the most common and used inquiry tools in dance.
However, I have noticed observation can provide more with collaborating with other inquiry tools.

Advantage;
this tool can provide more than what researcher can visually see, with researcher's deeper understanding toward the subject by using uses other inquiry tools before hand.
For example,  if observer, researcher choose to begin investigate the task by using tool of observation. Then, current event can provide only so much of information to the observer.
However, when observer has a clear aim and already investigated toward the subject before hand, i,e, certain dancers habit, teachers aim, or  what is the current event is focusing on, if researcher is already aware of these by using other inquiry tools, suddenly observation can provide different point of view.
If, researcher already investigated and correct some information by using other tools, it is much easier to understand the reason why things are happening and why current event is going to to certain direction. And, researcher can built a theory based on these evidences.
For example, I observed some dancer who have been interviewed by me, and, I saw some contradict act in the class from them. And, I also started to realise why dancers get paranoid about certain things, also how they get in to these mental state.
This tool could help put all the factors and show a bigger picture to researcher.

Detriment;
if, researcher has an fixed idea before using this tool, it could plants an assumption on researchers head; we see what we want to see. So, some times we are already deciding what researcher is going to see. That is what I found it tricky. Compare to rest of the tools, this tool is very much rely and depend on researchers understanding, as this is a very one way tool; there are technically no communication between researcher and participant.
And, often apparence of third party effect on participants mental state. And, that could give them  reasons to act differently to normal. In that case, observer is going to observe what participants want to observer to see, that beats the object. And, I could not find out how to avoid that situation.