title: Selection bias due to immigration in pharmacoepidemiologic studies.
abstract: OBJECTIVE
  To investigate the extent of selection bias due to exclusion of recent immigrants with respect to prevalence of antidiabetic drug use.
  
  
  METHODS
  Data on antidiabetic drug redemptions were obtained from Odense University Pharmacoepidemiologic Database for the period 1992-2003 covering a population of approximately 470,000. Ordinary prevalence estimates based on drug redemptions during a run-in period for non-immigrants were compared to estimates based on the waiting time distribution (WTD) approach. The WTD method was used to compare estimates for non-immigrants to estimates for the entire population.
  
  
  RESULTS
  When a run-in period of half a year was used and treatment cessation was accounted for, reasonable agreement between run-in estimates and WTD estimates was found, range of relative difference was -3.1% to 4.5%. Analyses using WTD method showed that prevalence among immigrants was approximately half that of non-immigrants in each of the years 1993-2003, resulting in a relative bias (non-immigrants vs. entire population) of approximately 1% for each year (range 0.73%; 1.36%). Stratifying on age and sex made the bias vanish, except for ages under 40.
  
  
  CONCLUSIONS
  Selection bias due to immigration, when studying prevalence of use of antidiabetics, is small after controlling for age and sex. The WTD approach is useful for studying selection bias and should potentially replace the use of run-in periods for determining treatment status.
authors: Research Unit of General Practice, University of Southern Denmark, Denmark. hstovring@health.sdu.dk
authors: Henrik
authors: H
authors: Støvring
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D003920:Diabetes Mellitus / Q000188:drug therapy / Q000453:epidemiology*; D004641:Emigration and Immigration; D005260:Female; D006801:Humans; D007004:Hypoglycemic Agents / Q000627:therapeutic use*; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D015983:Selection Bias; D013997:Time Factors
chemical_list: D007004:Hypoglycemic Agents