Early in an investigation a loose case definition is often used and tightened later. Why does the tighter definition matter for the analytic study?

1. A loose definition early on captures the extent of the problem and helps generate hypotheses while investigators are already in the field. 2. Once hypotheses are sharp, the study compares exposures between cases and non-cases. 3. People wrongly counted as cases dilute that comparison, so false positives can produce misleading results. 4. Investigators therefore tighten the definition, often dropping the possible and sometimes the probable category, before analysis. 5. A loose definition adds false positives rather than removing them, so the third option reverses the effect. 6. Consistency of application is required of any definition, loose or tight, so it does not distinguish them. _Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section More About Case Definitions_