title: Using linked educational attainment data to reduce bias due to missing outcome data in estimates of the association between the duration of breastfeeding and IQ at 15 years.
abstract: BACKGROUND
  Most epidemiological studies have missing information, leading to reduced power and potential bias. Estimates of exposure-outcome associations will generally be biased if the outcome variable is missing not at random (MNAR). Linkage to administrative data containing a proxy for the missing study outcome allows assessment of whether this outcome is MNAR and the evaluation of bias. We examined this in relation to the association between infant breastfeeding and IQ at 15 years, where a proxy for IQ was available through linkage to school attainment data.
  
  
  METHODS
  Subjects were those who enrolled in the Avon Longitudinal Study of Parents and Children in 1990-91 (n = 13 795), of whom 5023 had IQ measured at age 15. For those with missing IQ, 7030 (79%) had information on educational attainment at age 16 obtained through linkage to the National Pupil Database. The association between duration of breastfeeding and IQ was estimated using a complete case analysis, multiple imputation and inverse probability-of-missingness weighting; these estimates were then compared with those derived from analyses informed by the linkage.
  
  
  RESULTS
  IQ at 15 was MNAR-individuals with higher attainment were less likely to have missing IQ data, even after adjusting for socio-demographic factors. All the approaches underestimated the association between breastfeeding and IQ compared with analyses informed by linkage.
  
  
  CONCLUSIONS
  Linkage to administrative data containing a proxy for the outcome variable allows the MNAR assumption to be tested and more efficient analyses to be performed. Under certain circumstances, this may produce unbiased results.
authors: School of Social and Community Medicine, University of Bristol, Bristol, UK rosie.cornish@bristol.ac.uk.
authors: Rosie P
authors: RP
authors: Cornish
authors: School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors: Kate
authors: K
authors: Tilling
authors: School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors: Andy
authors: A
authors: Boyd
authors: School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors: Amy
authors: A
authors: Davies
authors: School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors: John
authors: J
authors: Macleod
mesh_terms: D000293:Adolescent; D015982:Bias; D001942:Breast Feeding / Q000706:statistics & numerical data*; D016208:Databases, Factual; D004522:Educational Status; D005260:Female; D006801:Humans; D016247:Information Storage and Retrieval; D007360:Intelligence / Q000502:physiology*; D016015:Logistic Models; D008137:Longitudinal Studies; D008297:Male; D017063:Outcome Assessment, Health Care; D010290:Parents; D011446:Prospective Studies; D011795:Surveys and Questionnaires
keywords: ALSPAC; Breastfeeding; IQ; administrative data; bias; linkage; missing data