1.5.3 measures prevalence

b

1. Measures - prevalence Prevalence Proportion of existing disease in the total population, without regard to when cases developed Numerator: number of existing cases of disease in the population Denominator: number of all persons in the population of interest A proportion Range is 0-1 - dimensionless Prevalence odds = prevalence of outcome/prevalence of no outcome = P/(1-P)

2. Measures - prevalence Two types of prevalence measures: Point prevalence: the proportion of subjects who have disease at a specified point in time – Example: proportion of population that is HIV positive on July 1, 2010

3. Measures - prevalence Two types of prevalence measures: Period prevalence: the proportion of subjects in a population who have disease during a certain period of time Uncommon - used when exact time of onset difficult to determine Example: proportion of population with an episode of depression over the past 12 months

4. Measures - prevalence Uses and limitations of prevalence A disease that has high incidence but is rapidly fatal or quickly cured would have low prevalence An exposure that increases survival with the disease will increase prevalence Useful for resource planning Can estimate the rate under certain conditions (more to come)

5. Measures - prevalence Uses and limitations of prevalence In measuring congenital anomalies we use prevalence out of necessity (many incident cases are lost, as are others in the denominator) – Cannot measure the population at-risk (conceptions) or person-time contributed by the population, so we necessarily take a point prevalence—the point being birth

6. Measures - prevalence Side note: Szklo and “prevalence rate” Szklo uses the term “prevalence rate” for prevalence Although you will see this in other places in the literature as well, you should not use this term Use the term prevalence Prevalence is not a rate and thus the term “prevalence rate” is incorrect and potentially confusing