Investigators tracing the source of a Salmonella outbreak want certainty that everyone studied truly had that infection. Which case definition suits them, and at what cost?

1. An investigator searching for a source wants no false positives, because people with diarrhea from another cause would blur the analysis. 2. That calls for a specific or strict case definition, which counts only confirmed infections with the organism. 3. A strict definition carries two costs: everyone with symptoms must be tested, and untested true cases are left out, so the total is underestimated. 4. A loose or sensitive definition does the opposite, capturing most true cases while sweeping in other illnesses, so both loose options misstate the trade-off. 5. The trap in the third option is real but belongs to the loose definition, not the strict one. 6. The cost of a strict definition is undercounting, not overcounting. _Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Variation in Case Definitions_