Successful epidemiological studies require sound design, sensible analysis, and careful execution. Epidemiologists ensure the quality of their fieldwork in many ways. They seldom collect data without written, standard instruments designed to increase comparability between the study groups. Typical studies use a variety of instruments: abstracts from medical records, personal interview questionnaires, self-administered questionnaires, telephone interview questionnaires, physical examinations, biological specimen collections, and environmental samples.46
The first step in any outbreak investigation is to evaluate clinical and laboratory findings to confirm that a disease outbreak has occurred. The next step is to construct a case definition so that the number of cases and the attack rate can be determined. A case is the person or animal in the population or group identified as having the particular disease, or condition under investigation.6 A variety of criteria may be used to identify cases. Examples of clinical criteria are physician diagnoses, abstracts of clinical records, and a summary of similar clinical findings. Epidemiologic criteria might be exposure history— such as exposure to other cases of a disease or animal vectors—or consumption of a suspected food vehicle. A laboratory criterion would be the accepted diagnostic method for identification of the known etiologic agent. Suspected cases will meet either clinical or epidemiologic criteria, and a definite case usually requires laboratory confirmation.
Accurate development of a case definition is especially important in a potential bioterrorism event, since there may well be an increased potential for psychological consequences with acute psychosomatic symptoms. Such signs and symptoms may be confused with infection or intoxication.47
Calculation of the attack rate allows investigators to compare the estimated rate of illness with rates during previous time periods to determine whether the rate constitutes a deviation from the norm. The attack rate or case rate is a cumulative incidence rate (the proportion of a group of people who experience the onset of disease) used for a particular group observed for limited periods under special circumstances such as an epidemic.6
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