title: Multiple imputation for assessment of exposures to drinking water contaminants: evaluation with the Atrazine Monitoring Program.
abstract: BACKGROUND
  Drinking water may contain pollutants that harm human health. The frequency of pollutant monitoring may occur quarterly, annually, or less frequently, depending upon the pollutant, the pollutant concentration, and community water system. However, birth and other health outcomes are associated with narrow time-windows of exposure. Infrequent monitoring impedes linkage between water quality and health outcomes for epidemiological analyses.
  
  
  OBJECTIVE
  To evaluate the performance of multiple imputation to fill in water quality values between measurements in community water systems (CWSs).
  
  
  METHODS
  The multiple imputation method was implemented in a simulated setting using data from the Atrazine Monitoring Program (AMP, 2006-2009 in five Midwestern states). Values were deleted from the AMP data to leave one measurement per month. Four patterns reflecting drinking water monitoring regulations were used to delete months of data in each CWS: three patterns were missing at random and one pattern was missing not at random. Synthetic health outcome data were created using a linear and a Poisson exposure-response relationship with five levels of hypothesized association, respectively. The multiple imputation method was evaluated by comparing the exposure-response relationships estimated based on multiply imputed data with the hypothesized association.
  
  
  RESULTS
  The four patterns deleted 65-92% months of atrazine observations in AMP data. Even with these high rates of missing information, our procedure was able to recover most of the missing information when the synthetic health outcome was included for missing at random patterns and for missing not at random patterns with low-to-moderate exposure-response relationships.
  
  
  CONCLUSIONS
  Multiple imputation appears to be an effective method for filling in water quality values between measurements.
authors.affiliation: Division of Environmental and Occupational Health Sciences, School of Public Health, University of Illinois at Chicago, 2121 W. Taylor St. (M/C 922), Chicago, IL 60612, United States. Electronic address: rjones25@uic.edu.
authors.forename: Rachael M
authors.initials: RM
authors.lastname: Jones
authors.affiliation: Division of Epidemiology and Biostatistics, School of Public Health, University of Illinois at Chicago, United States.
authors.forename: Leslie T
authors.initials: LT
authors.lastname: Stayner
authors.affiliation: Division of Epidemiology and Biostatistics, School of Public Health, University of Illinois at Chicago, United States.
authors.forename: Hakan
authors.initials: H
authors.lastname: Demirtas
mesh_terms: D001280:Atrazine / Q000032:analysis / Q000633:toxicity*; D060766:Drinking Water / Q000737:chemistry*; D004781:Environmental Exposure; D004784:Environmental Monitoring; D006540:Herbicides / Q000032:analysis / Q000633:toxicity*; D015146:Midwestern United States; D014874:Water Pollutants, Chemical / Q000032:analysis / Q000633:toxicity*
chemical_list: D060766:Drinking Water; D006540:Herbicides; D014874:Water Pollutants, Chemical; D001280:Atrazine
keywords: Atrazine; Atrazine Monitoring Program; Drinking water quality; Multiple imputation