Many dairy scientists are not familiar with epidemiologic study designs and measures. The various designs (cross-sectional, case-control, cohort) are reviewed with special reference to their strengths and weaknesses. Epidemiologic measures (incidence and prevalence rates, relative risk, odds ratio, attributable fraction, and population attributable fraction) are defined, and their validity in the contexts of the different study designs is discussed. Incidence rate (risk of disease) is the number of new cases of a disease divided by the population at risk during a specified time period. Prevalence rate is the frequency of both new and old cases in a population at a specified point or period in time divided by the average population during that same period. Both incidence and duration of disease contribute to prevalence. Odds ratios and relative risks are measures of association (how much disease the exposed group experienced relative to the nonexposed group) and are used to assess the relative importance of risk factors. Attributable fraction is the proportion of the incidence in the exposed group that can be attributed to the specific exposure. Population attributuable fraction is the expected reduction in incidence in the whole population if the specific exposure were prevented. The attributable fractions are useful in planning preventive and herd health programs.

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