It is regarding a serial comma (also called Oxford comma and Harvard comma).
I am following APA style which recommends using a comma between elements (including before and and or) in a series of three or more items.
My query is this: In a single report, can i use this way only when there is ambiguity? OR Should I make it consistent and use this in all the places?
All things being equal, it makes sense to use it consistently throughout a single report. I'm not sure what kind of ambiguity you're referring to, but changing your comma usage within a single writing project seems more likely to create ambiguity than to alleviate it. The usage dictated by style guides is not generally treated on an "opt in/opt out" basis.
Last edited by iannou; 21-Feb-2013 at 09:03. Reason: typo