title: A comparison of missing data procedures for addressing selection bias in HIV sentinel surveillance data.
abstract: BACKGROUND
  Selection bias is common in clinic-based HIV surveillance. Clinics located in HIV hotspots are often the first to be chosen and monitored, while clinics in less prevalent areas are added to the surveillance system later on. Consequently, the estimated HIV prevalence based on clinic data is substantially distorted, with markedly higher HIV prevalence in the earlier periods and trends that reveal much more dramatic declines than actually occur.
  
  
  METHODS
  Using simulations, we compare and contrast the performance of the various approaches and models for handling selection bias in clinic-based HIV surveillance. In particular, we compare the application of complete-case analysis and multiple imputation (MI). Several models are considered for each of the approaches. We demonstrate the application of the methods through sentinel surveillance data collected between 2002 and 2008 from India.
  
  
  RESULTS
  Simulations suggested that selection bias, if not handled properly, can lead to biased estimates of HIV prevalence trends and inaccurate evaluation of program impact. Complete-case analysis and MI differed considerably in their ability to handle selection bias. In scenarios where HIV prevalence remained constant over time (i.e. β = 0), the estimated β^1 derived from MI tended to be biased downward. Depending on the imputation model used, the estimated bias ranged from -1.883 to -0.048 in logit prevalence. Furthermore, as the level of selection bias intensified, the extent of bias also increased. In contrast, the estimates yielded by complete-case analysis were relatively unbiased and stable across the various scenarios. The estimated bias ranged from -0.002 to 0.002 in logit prevalence.
  
  
  CONCLUSIONS
  Given that selection bias is common in clinic-based HIV surveillance, when analyzing data from such sources appropriate adjustment methods need to be applied. The results in this paper suggest that indiscriminant application of imputation models can lead to biased results.
authors.affiliation: Institute for Health Metrics and Evaluation, University of Washington, Seattle, USA. marieng@uw.edu.
authors.forename: Marie
authors.initials: M
authors.lastname: Ng
authors.forename: Emmanuela
authors.initials: E
authors.lastname: Gakidou
authors.forename: Christopher Jl
authors.initials: CJ
authors.lastname: Murray
authors.forename: Stephen S
authors.initials: SS
authors.lastname: Lim