In the typical quantitative study, category or variable definition precedes data collection. In contrast, participant observers have only general questions as their guides and cannot translate them into standardized measures. As a result, the qualitative researcher both observes and chooses what do observe at the same time.
The alternating process of observing and defining categories will repeat throughout the study and helps to avoid blinding preconceptions and rigid measurement procedures. The flexibility and sensitivity of this approach may allow profound penetration of the subject matter. As counterparts to these advantages, this method faces the threats of unreliable measures, instrumentation shift, and observer bias.