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Learning ObjectivesTo define and provide examples of a populationTo distinguish between a fixed and dynamic populationTo describe the key aspects of measuring disease occurrenceTo define and distinguish among prevalence, cumulative incidence, and incidence rateTo describe the mathematical relationship among the measures of disease frequencyTo calculate and interpret commonly used measures of disease frequency in public healthTo know how to calculate person-timeTo organize disease frequency data into a two-by-two tableTo describe and calculate absolute and relative measures of comparison including rate/risk difference, population rate/risk difference, attributable proportion between the exposed and total population, and the rate/risk ratio (relative risk)To be able to interpret each absolute and relative measure of comparisonTo describe the purposes of rate standardization and calculate the directly- adjusted standardized rate, and SMRs using indirect standardizationdisease frequency in populationsthrough the calculation of rates, we have to actually measure the frequency of disease in a population before we can look at its distribution geographically or by other factorshelps identify patterns of disease and also disease trends in a population as they occurshort-term and long-term trends are all based upon the calculation of ratesA population in general is a group of persons with a common characteristic range from living in the same city, to being born in the same year, to being in the same classroomthere are some rates that are more appropriate to a fixed population and some rates more appropriate to a dynamic populationfixed populationonce you're in that population, you're in it forever until you essentially exit it through deathExamples members of the Framingham cohort study of Massachusetts, persons who were living around the Chernobyl reactor in Ukraine in 1986 and were exposed to radiation, and also individuals living in Hiroshima and Nagasaki, Japan, and were exposed to the atomic bombings inboth cities. Their exposure fixed them in those two populations foreverdynamic populationmore commonpeople enter and leave - birth, deaths, migration, immigration. It's in constant flux/changingoEx. the population of most countries or most cities or most states are defined as dynamic populationsthe mechanics of actually how we calculate rates in a populationneed three pieces of information
1.a numerator, the number of cases of a particular disease that occurs in a population2.a number of the population itself, or a denominator3.need to account for that time to the calculation of person time - in the case of the only true rate,an incidence rate, a function of time, or a person time, because the rate of disease changes as a person moves through timeThree Types of Frequency MeasuresUsed to relate number cases of disease, size of population, and time.