title: Sampling and measurement methods for a study of childhood refractive error in a UK population.
abstract: BACKGROUND
  There is a paucity of data describing the prevalence of childhood refractive error in the United Kingdom. The Northern Ireland Childhood Errors of Refraction study, along with its sister study the Aston Eye Study, are the first population-based surveys of children using both random cluster sampling and cycloplegic autorefraction to quantify levels of refractive error in the United Kingdom.
  
  
  METHODS
  Children aged 6-7 years and 12-13 years were recruited from a stratified random sample of primary and post-primary schools, representative of the population of Northern Ireland as a whole. Measurements included assessment of visual acuity, oculomotor balance, ocular biometry and cycloplegic binocular open-field autorefraction. Questionnaires were used to identify putative risk factors for refractive error.
  
  
  RESULTS
  399 (57%) of 6-7 years and 669 (60%) of 12-13 years participated. School participation rates did not vary statistically significantly with the size of the school, whether the school is urban or rural, or whether it is in a deprived/non-deprived area. The gender balance, ethnicity and type of schooling of participants are reflective of the Northern Ireland population.
  
  
  CONCLUSIONS
  The study design, sample size and methodology will ensure accurate measures of the prevalence of refractive errors in the target population and will facilitate comparisons with other population-based refractive data.
authors.affiliation: School of Biomedical Sciences, University of Ulster, Cromore Road, Coleraine BT52 1SA, UK. l.odonoghue@ulster.ac.uk
authors.forename: L
authors.initials: L
authors.lastname: O'Donoghue
authors.forename: K J
authors.initials: KJ
authors.lastname: Saunders
authors.forename: J F
authors.initials: JF
authors.lastname: McClelland
authors.forename: N S
authors.initials: NS
authors.lastname: Logan
authors.forename: A R
authors.initials: AR
authors.lastname: Rudnicka
authors.forename: B
authors.initials: B
authors.lastname: Gilmartin
authors.forename: C G
authors.initials: CG
authors.lastname: Owen
mesh_terms: D000293:Adolescent; D002648:Child; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D008297:Male; D009660:Northern Ireland / Q000453:epidemiology; D010808:Physical Examination / Q000379:methods; D012030:Refractive Errors / Q000453:epidemiology*; D012107:Research Design; D014787:Vision Tests