2.1.2. Cohort studies
Cohort studies allow an investigator to estimate the disease incidence rate because the study measures the time that participants don’t have the disease. As compared to cross-sectional studies, cohort studies are better able to assess causality because the temporal relationship of exposure preceding outcome is not subject to question. This design is implemented through three steps. First, exposed and unexposed individuals who are free of the outcome of interest are identified and become the cohort. Next, each cohort is observed for a minimum period of time to determine if the outcome of interest develops. The risk of developing the outcome is calculated separately for the exposed group and for the unexposed group. Finally, the risk for the exposed and unexposed study subjects is compared, often by estimating the relative risk. Essentially, the incidence of disease over time is measured in exposed and unexposed individuals to determine the risk of disease in relation to exposure to a factor of interest. These studies can be performed retrospectively, where a post-hoc study is executed on previously collected data, or prospectively, where study subjects who do not have the outcome of interest are followed forward through time. Examples of cohort studies in honey bees include Genersch et al. (2010), Gisder et al. (2010) and vanEngelsdorp et al. (2013).