I will always be in favor of narrowing a study. It's easier. Keep it tight. If you expand and broaden your study, the work becomes unmanageable. Do not mistake narrow with insufficient or incomplete. The subject must be completely addressed. An example: I may generally be researching about navigation and I have a proposal to improve GPS navigation. I have to briefly mention visual, compass, celestial navigation and other electronic navigation, but I must present everything about GPS navigation and how it applies to my study in the review of related literature.I do not need to talk about Polynesian star-stick navigation but all research done to improve GPS navigation since GPS was developed (and especially within the last five years) has to be mentioned in the review.