title: A method for meta-analysis where the level of data aggregation differs between studies.
abstract: BACKGROUND
  This paper presents a statistical method that was used to assess haemoglobin levels world-wide among women, based on a combination of over 400 studies.
  
  
  METHODS
  The methodology is easy to implement and is specifically adapted to the case where mean observations are taken, although the subgroups represented by the mean values may differ from study to study. That is, the level of data aggregation is not consistent between studies.
  
  
  RESULTS
  In this example some studies report average haemoglobin levels for a sample of the population, while others give averages by urban/rural classification and/or pregnancy status. Though the method is based on likelihood principles, computation is straightforward.
  
  
  CONCLUSIONS
  The methodology can be applied to a variety of meta-analytic situations where assessments are based on combining data from several sources. We provide an example of how the method was implemented in a study of haemoglobin levels among women undertaken at the World Health Organisation.
authors.affiliation: Department of Statistics, Brigham Young University, Provo, UT 84602, USA.
authors.forename: G W
authors.initials: GW
authors.lastname: Fellingham
authors.forename: Hd
authors.initials: H
authors.lastname: Tolley
authors.forename: M
authors.initials: M
authors.lastname: Anker
authors.forename: E
authors.initials: E
authors.lastname: Ahman
mesh_terms: D003625:Data Collection / Q000379:methods*; D005260:Female; D006454:Hemoglobins / Q000032:analysis*; D006801:Humans; D015201:Meta-Analysis as Topic; D015233:Models, Statistical; D011247:Pregnancy; D012424:Rural Population / Q000706:statistics & numerical data; D014505:Urban Population / Q000706:statistics & numerical data; D014944:World Health Organization
chemical_list: D006454:Hemoglobins