title: Design, methodological issues and participation in a multiple sclerosis case-control study.
abstract: OBJECTIVE
  This study was conducted to determine whether the risk of developing multiple sclerosis (MS) was associated with certain environmental exposures or genetic factors previously reported to influence MS risk. This paper describes the methodological issues, study design and characteristics of the study population.
  
  
  METHODS
  Individuals with definite MS were identified from a prevalence study conducted in three geographic areas. The target number of cases was not reached, so an additional study area was added. Identifying clinic controls was inefficient, so controls were recruited using random digit dialing. All study participants completed a detailed questionnaire regarding environmental exposures using computer-assisted telephone interviewing, and blood was collected for genetic analysis.
  
  
  RESULTS
  In total, 276 cases and 590 controls participated, but participation rates were low, ranging from 28.4% to 38.9%. Only one-third (33.6%) of individuals identified in the prevalence study agreed to participate in the case-control study. Cases were more likely to be non-Hispanic white and older than their source populations as identified in the preceding prevalence study (P < 0.05). Most participants provided a blood sample for genotyping (91%; n = 789).
  
  
  CONCLUSIONS
  Epidemiological studies play a key role in identifying genetic and environmental factors that are associated with complex diseases like MS. Methodological issues arise in every study, and investigators need to be able to detect, respond to and correct problems in a timely and scientifically valid manner.
authors.affiliation: Division of Reproductive Health, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA. djw8@cdc.gov
authors.forename: D M
authors.initials: DM
authors.lastname: Williamson
authors.forename: R A
authors.initials: RA
authors.lastname: Marrie
authors.forename: A
authors.initials: A
authors.lastname: Ashley-Koch
authors.forename: R
authors.initials: R
authors.lastname: Schiffer
authors.forename: J
authors.initials: J
authors.lastname: Trottier
authors.forename: L
authors.initials: L
authors.lastname: Wagner
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D016022:Case-Control Studies; D003625:Data Collection / Q000706:statistics & numerical data; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000453:epidemiology* / Q000209:etiology* / Q000235:genetics; D012107:Research Design; D018401:Sample Size; D057566:Self Report; D012737:Sex Factors; D011795:Surveys and Questionnaires; D055815:Young Adult