The role of structure
An alternative approach is to use the concept of structure - since the different methods in social research can be classified in terms of how much structure is employed in the research instruments.
Notes:
Since the qualitative/quantitative distinction can be rather unhelpful when talking about methods (although the distinction may be very useful when talking of methodology), I have evolved a typology based on how the structure of the data is determined - how much structure is provided, and when.