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Why do epidemiologists create a case definition before calculating an attack rate?

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Know the investigations of researchers using genomic information to reconstruct the cause and transmission routes of the bubonic plague and the Black Death — epidemiology Researchers using genomic information to trace the transmission routes in past epidemics of plague.

In epidemiology, a case definition is essential because it provides the standardized criteria required to identify who should be counted as a "case" of a specific disease or health event.12

When calculating an attack rate, which measures the proportion of a population that becomes ill, the case definition serves two critical functions:

Defining the Numerator: It establishes the specific clinical, laboratory, or situational characteristics that determine whether an individual is included in the count of those who became ill.12
Ensuring Consistency: By providing a clear, uniform set of criteria, it allows researchers to accurately distinguish between those who meet the definition and those who do not, ensuring that the resulting rate is reliable and comparable across different groups or regions.1

Without a clear case definition, it would be impossible to consistently identify the individuals who constitute the "ill" portion of the population, thereby preventing an accurate calculation of the attack rate.2