title: The relationship between time since registration and measured incidence rates in the General Practice Research Database.
abstract: BACKGROUND
  The General Practice Research Database (GPRD) is widely used to study incidence rates. This study examines whether incidence rates are overestimated during the first year after registration, how long one needs to wait to obtain accurate incidence rates, and whether the time period of overestimation differs among disease types.
  
  
  METHODS
  We measured incidence rates of nine acute, eight chronic, and eight neoplastic outcomes in 3-month intervals through month 36 after enrollment in GPRD. The incidence rates in months 13-36 were used to estimate baseline incidence rates for each diagnosis.
  
  
  RESULTS
  For patients registering with practices that were already UTS, incidence rates were highest in the first 3 months after registration. In eight of nine acute diagnoses, the incidence rate was within 20% of baseline by months 4-6; and in seven of eight cancers, the incidence rate was within 20% of baseline by months 7-9. For chronic conditions, the incidence rate in months 10-12 differed from baseline by more than 20% for five of the eight outcomes, respectively. For patients registering prior to UTS, incidence rates during the first quarter were within 20% of baseline for all acute and cancer diagnoses and six of eight chronic diagnoses.
  
  
  CONCLUSIONS
  Reported incidence rates are highest in the first 3 months after registration with an UTS practice and decline to baseline over the first year, more quickly for acute conditions than chronic conditions. For patients who registered prior to UTS, incidence rates are near the baseline level at the start of follow-up.
authors.affiliation: Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA, USA. jlewis@cceb.med.upenn.edu
authors.forename: James D
authors.initials: JD
authors.lastname: Lewis
authors.forename: Warren B
authors.initials: WB
authors.lastname: Bilker
authors.forename: Rachel B
authors.initials: RB
authors.lastname: Weinstein
authors.forename: Brian L
authors.initials: BL
authors.lastname: Strom
mesh_terms: D015331:Cohort Studies; D015986:Confounding Factors, Epidemiologic; D016208:Databases, Factual / Q000706:statistics & numerical data*; D005194:Family Practice / Q000706:statistics & numerical data*; D006801:Humans; D015994:Incidence; D016347:Medical Records Systems, Computerized; D012189:Retrospective Studies; D013997:Time Factors; D006113:United Kingdom / Q000453:epidemiology