title: Evidence synthesis combining individual patient data and aggregate data: a systematic review identified current practice and possible methods.
abstract: OBJECTIVE
  Meta-analysis of individual patient data (IPD) is the "gold-standard" for synthesizing evidence across several studies. Some studies, however, may only provide aggregate data (AD). In this situation researchers might need to combine IPD with AD to utilize all the evidence available. Here, we review applied IPD meta-analysis articles to assess if and how AD is combined with IPD in practice.
  
  
  METHODS
  A systematic review of articles identified from bibliographic databases and searches.
  
  
  RESULTS
  We identified 33 applied IPD articles that combined IPD and AD and 166 that did not. For each article, we recorded the proportion of total studies providing IPD, and found that articles combining IPD and AD had, on average, IPD available in only 64% of studies (compared to 90% in articles not combining IPD and AD). Two different methods were used to combine IPD and AD, the two-stage method and analysis of partially reconstructed IPD, but a review of methodological articles identified two further methods, multilevel modeling and Bayesian hierarchical related regression. We summarize each method to aid practitioners.
  
  
  CONCLUSIONS
  Combining IPD and AD is a relevant issue for evidence synthesis, and the further development and validation of suitable meta-analysis methods is needed.
authors.affiliation: Centre for Medical Statistics and Health Evaluation, School of Health Sciences, University of Liverpool, UK. richard.riley@liv.ac.uk
authors.forename: Richard D
authors.initials: RD
authors.lastname: Riley
authors.forename: Mark C
authors.initials: MC
authors.lastname: Simmonds
authors.forename: Maxime P
authors.initials: MP
authors.lastname: Look
mesh_terms: D001499:Bayes Theorem; D003625:Data Collection / Q000379:methods*; D003627:Data Interpretation, Statistical; D015340:Epidemiologic Research Design; D004813:Epidemiology; D019317:Evidence-Based Medicine / Q000379:methods; D006801:Humans; D015233:Models, Statistical