A set of training materials for professionals working in intervention epidemiology, public health microbiology and infection control and hospital hygiene.
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Jean Claude Desenclos
In Romans' time, a cohort of legionnaires consisted of
a group of soldiers sharing the same military events for a certain
time. In epidemiology we consider that a cohort consists of people
the same population and sharing similar experience for a defined period
Cohort studies involve the comparison of disease
incidence over time (risk or rate) between two subsets of a population
cohorts). One of the 2 cohorts is exposed to a certain characteristic
(exposure). The other is not. All other things being equal between the
cohorts but for their exposure. In both cohorts we measure occurrence of
disease over the specific study period. However whenever the condition
other things being equal" is not met, the comparison might be wrong.
The following graph adapted from Rodrigues  illustrates
occurrence of cases over time in the two cohorts. Initially Ne persons
exposed and Nu persons are unexposed. The number of persons who are
free decreases over time (shaded area). The number of cases (non shaded
increases over time but more in the exposed cohort. At the end of the
respectively Ce and Cu cases have occurred in the exposed and unexposed
The shaded area represents the cumulative time during which persons were
risk of developing disease in each of the cohorts during the entire
 Rodrigues L. Kirkwood BR. Case-control
designs in the study of common diseases: updates on the demise of the
rare disease assumption and the choice of sampling scheme for controls.
Int J Epidemiol 1990 Mar:19(1):205-13
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Arnold Bosman posted on 4/26/2014 7:52:35 PM:
So here we see that the relative risk is synonymous to risk-ratio. Funny enough, we do not speak of a 'relative rate', yet only of a rate-ratio.
sbpmebxu replied on 7/29/2015 7:25:30 PM: 1
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