![]() How do we do this? By defining the "at risk" period for each person in the cohort. At first blush, the exposure can be operationalized by simply counting the number of occupied beds for each person's length of stay, and taking the average, repeated for each person in the cohort.īut wait: the exposure must precede the outcome. The outcome is straightforward to operationalize: people will either have a diagnosed infection (outcome) or not (right censored). We hypothesize that a higher average number of occupied beds will confer a greater risk for infection. To make this more concrete, assume a cohort is formed to assess new cases of healthcare acquired infections in a hospital and the exposure is the average number of occupied beds during the patient's length of stay (perhaps mean or median dichotomized so it can be assessed as an exposure/unexposed predictor). How do we operationalize the exposure ensuring temporality, and that the "at risk" period for each individual is correctly analyzed? In a retrospective study, we already have knowledge of the outcome. In a prospective study, the cohort is recruited on the basis of the exposure, hopefully ensuring some degree of temporality. Take the above hypothetical cohort as an example. This blog post serves as a caveat for those conducting a retrospective cohort. Not because the exposure has not preceded the outcome, but because more care needs to be given when operationalizing the exposure. In a retrospective cohort, ensuring temporality becomes more of a challenge. By its very nature the cohort is formed on the basis of an exposure, without knowledge of the eventual outcome (although there may be a suspicion as the basis of the hypothesis). In a prospective cohort, ensuring temporality is easier (let's ignore outcomes with long incubation or latent periods). Ideally ensured in the study's design is the notion of temporality exposure preceding the outcome offers greater evidence of causality. Goldstein, PhD, MBI About | Blog | Books | CV | DataĪssessing exposure in retrospective cohortsĪ cohort is usually formed in an epidemiological study to assess the relationship of an antecedent exposure to an incident outcome. Goldstein, PhD, MBI - Assessing exposure in retrospective cohorts Neal D.
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