Transcription of Calculating Prevalences and Incidences
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Calculating Prevalences and Incidences
Page 1 of C:\DATA\HS161\ Prevalences and = no. of casespopulation can be measured in an closed cohort or in an open in Old cases and new cases are counted in the be measured at a particular point (point prevalence ) or over a period (period prevalence ). Normally,when we say prevalence we mean point prevalence . proportion = Cumulative incidence = Risk = no. of disease onsetsno. initially at proportion can only be measured in a closed new onsets are time of follow-up must be specified ( , 5-year risk of breast cancer vs. lifetime risk of breast cancer). rate = incidence density = no. of disease onsetsSum of person-time @ rates (density) can be measured in a closed cohort or in an open numerator is the same as incidence proportion, but its denominator is different. of Calculating the person-time a closed cohort(1)Count person-time for each individual in the cohort and sum ( , see text Fig , p. 100 for anexample).(2)Break cohort into those who remain healthy (Group 1) and those who develop disease (Group 2).
Page 1 of C:\DATA\HS161\pre-inc-key-points.wpd Calculating Prevalences and Incidences 1. Prevalence = no. of cases population size a. Prevalence can be measured in an closed cohort or in an open population.
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