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## Point Prevalence vs. Period Prevalence

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**CDC Site Visit at EmoryCHD Surveillance Cooperative**Agreement Data Analysis:Point vs. Period Prevalence & Capture-RecaptureSeptember 25, 2013Carol Hogue, PhD, MPHCheryl Raskind-Hood, MS, MPH**Point Prevalence vs. Period Prevalence**For a given interval, the period prevalence equals the point prevalence at the beginning of the interval plus the incidence (new cases) during the interval. • Point Prevalence: How much CHD is present in the population at a particular point in time or date? # CHD cases on a specific date / # of people in the population on this date • Period Prevalence: How much CHD is present in the population over a period of time? # CHD cases over a specific time period / # of people in the population during same time period • For instance, it’s calculated same as point prevalence, except the numerator is the # of people who had CHD at any time during a specified time period e.g., 2008-2010. • When comparing prevalence rates: • Need to ensure that the length of the time period is the same. You cannot compared a point prevalence on Jan 1, 2010 with a period prevalence for 2008-2010. • Prevalence rates may be compared among different diseases or populations as long as the same length of time is used.**“Correcting” Estimates**• Virtually all CHD healthcare for five-county residents will be included in one or more data captures • Possible to conduct “capture / re-capture” analyses to estimate missing cases not under care • For example, ResDAC with clinic records**Simple Capture/Recapture Model**• From Sekar & Deming (1949)* originally developed to estimate births and deaths in India • Estimate the number (Y) of adolescent/adult CHD cases not found in either clinic (A) or Medicaid (B) systems by Y = (A1B1)/C Where: C = cases recorded in both systems A1 = cases recorded in clinic records only B1 = cases recorded in Medicaid only *Sekar CC, Deming WE. On a method of estimating birth and death rates and the extent of registration. Journal of the American Statistical Association. 1949;44(245):101-115.**Capture-RecaptureMethodology**Cases of CHD found in both datasets Cases of CHD found in Adolescent dataset only MACDP Yes No 1653 Yes 423 Adolescent CHD db x No 93 Cases of CHD found in MACDP dataset only Missed Cases of CHD estimated to equal b*c / a i.e., 423*93/1653 = 24 Total estimated CHD cases (a+b+c+d) = 2193**Point Estimate (N) of the Total Number of Prevalent Cases**of Adolescent and Adult CHD N = A1 + B1 + C + (A1B1/C) Approximate estimate of the variance, V(N) is given by: V(N) = Nq1q2/p1p2 Where p1 = C+A1/N; p2 = C+B1/N and p1 + q1 = p2 + q2 = 1**Modeling Capture/Recapture**• Loglinearmodels to estimate the prevalence can account for the probability of being on a given list varying from individual to individual through interaction terms. • When the models are analyzed within socioeconomic or racial / ethnic categories (e.g., age & race / ethnicity), this analytic strategy can also be used to estimate disparities in healthcare.**Capture/Recapture EstimationDatasets Available for**Prevalence Estimate * Corrected for deaths from NDI ** Partially corrected for deaths from NDI *** For special population of pregnant women