Impact numbers: measures of risk factor impact on the whole population from case-control and cohort studies.
OBJECTIVE
To describe new measures of risk from case-control and cohort studies, which are simple to understand and relate to numbers of the population at risk.


METHODS
Theoretical development of new measures of risk.


METHODS
Review of literature and previously described measures.


RESULTS
The new measures are: (1) the population impact number (PIN), the number of those in the whole population among whom one case is attributable to the exposure or risk factor (this is equivalent to the reciprocal of the population attributable risk); (2) the case impact number (CIN) the number of people with the disease or outcome for whom one case will be attributable to the exposure or risk factor (this is equivalent to the reciprocal of the population attributable fraction); (3) the exposure impact number (EIN) the number of people with the exposure among whom one excess case is attributable to the exposure (this is equivalent to the reciprocal of the attributable risk); (4) the exposed cases impact number (ECIN) the number of exposed cases among whom one case is attributable to the exposure (this is equivalent to the reciprocal of the aetiological fraction). The impact number reflects the number of people in each population (the whole population, the cases, all those exposed, and the exposed cases) among whom one case is attributable to the particular risk factor.


CONCLUSIONS
These new measures should help communicate the impact on a population, of estimates of risk derived from cohort or case-control studies.
Evidence for Population Health Unit, The Medical School, The University of Manchester, UK. Dick.heller@man.ac.uk
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D016022:Case-Control Studies; D015331:Cohort Studies; D004194:Disease / Q000209:etiology*; D004812:Epidemiologic Methods; D006801:Humans; D015994:Incidence; D018570:Risk Assessment / Q000379:methods*; D012307:Risk Factors; D009626:Terminology as Topic