This is a method of comparing the incidence rates of disease in exposed and non-exposed populations. The SMR is a product of convenience. In this calculation, the control group of a cohort study to only 'exist' on paper and is calculated from national statistics tables. The rates undergo age standardization and then the SMR is calculated:
SMR = (observed deaths in study population / expected deaths in study population) x100
The 'expected deaths' are the number of deaths that would occur if the study population experienced the same age-specific mortality as the reference population. Thus if the SMR is < 100, the mortality experience of the study population is less that the reference population.
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