On the other hand - do you really need research / theoretical texts for this? I don't know what your proposal is on, but perhaps it is enough to say, for example: "people have different personalities and some may be less confidence and/or less verbose than others, and therefore, I would moderate the focus groups to ensure that one or two persons did not dominate the discussion".
I have no clue what your research is on - but it may be that you can get away with a sentence like that if it's just a little consideration you need to make for your project (as opposed to something theoretically relevant to your ideas/project).
Hope this helps!
Tudor