I'm a doctoral student doing research in the humanities. As part of my research I have gathered together a lot of interview text. To analyse this data I want to be able to easily tag sections of text with keywords (the tags need to be able to overlap, and perhaps be organised hierarchically) and later be able to collate those sections from across multiple files. I need to be able to do this on a Mac.

It feels like a simple task but I can't find any software for doing it that isn't either horribly clunky or a massive overkill worth hundreds of pounds. Is there any good software for doing this, or are there any good ways of doing it with other software?