The temporal-geographic signal that motivated the CDC investigation.
Methodology
CDC (Enteric Diseases Branch) descriptive outbreak-surveillance study. Sources: regional foodborne-outbreak reports for the US Northeast (Jan 1985-May 1987), national Salmonella outbreak surveillance (1973-1984) comparing egg-association across serotypes, and microbiological cultures of bulk raw eggs from pasteurization plants by region. Design is observational surveillance (no experimental control); the causal reading rests on convergent, replicated signals.
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