Structure - imposed or emergent
All research methods involve structure to some extent - even anthropological field notes have some initial struture, based on the research objectives.
Key point - whether the structure is imposed in its totality by the researcher, or whether the structure emerges from the research process.
Structure now enters the classification. No data collection process can be totally lacking in structure - we will always have some ideas, either derived from prior knowledge of the situation, or from the person we are approaching to interview, or derived from prior research or theory, or simply from our research objectives. We may wish to put all of that aside in, say, interviewing, but we cannot possibly take it out of mind completely.
The key point is whether the structure is imposed in its totality by the researcher, for example, in designing a self-completed questionnaire, or whether the structure emerges from the research process; for example, by analysing interview transcripts and developing a conceptual structure in the process.