title: Profound effect of study design factors on ventilator-associated pneumonia incidence of prevention studies: benchmarking the literature experience.
abstract: BACKGROUND
  The ventilator-associated pneumonia incident proportion (VAP-IP) is highly variable among control groups of studies of methods for its prevention. The objective here is to develop and validate a literature-derived benchmark against which these groups can be profiled.
  
  
  METHODS
  A literature search yielded 95 cohort groups and control and intervention groups of 150 studies of either non-antimicrobial or antimicrobial methods of VAP prevention. The 95 cohort groups comprise a benchmark set (30 groups), from which the reference funnel plot (RFP) was derived, and a search set (65 groups), against which the benchmark was validated. The VAP-IP data of the benchmark set were found in five published systematic reviews, whereas the VAP-IP data of the search set were abstracted directly from the literature.
  
  
  RESULTS
  Among the 95 cohort groups, the VAP-IP of groups with size >399 was significantly lower than the VAP-IP of smaller groups. Compared with the RFP, 15 of 51 (29%) control groups from studies of antimicrobial methods of VAP prevention with concurrent design were high outlier versus 2 of 110 (2%) control groups from other types of study design (P < 0.001). There were only 22 (14%) outlier groups, all low outlier, among the 162 intervention groups.
  
  
  CONCLUSIONS
  Study design factors such as concurrency and study size have potentially greater influence on the VAP-IP than do the VAP prevention methods under study. The outlier status of control groups were inapparent in the individual studies and the meta-analyses and yet would have confounded the estimates of treatment effect.
authors.affiliation: School of Rural Health, University of Melbourne, Australia. jamesh@bhs.org.au
authors.forename: James C
authors.initials: JC
authors.lastname: Hurley
mesh_terms: D000900:Anti-Bacterial Agents / Q000627:therapeutic use; D019985:Benchmarking; D002986:Clinical Trials as Topic; D006801:Humans; D015994:Incidence; D053717:Pneumonia, Ventilator-Associated / Q000188:drug therapy / Q000517:prevention & control*; D012107:Research Design; D012121:Respiration, Artificial / Q000009:adverse effects*
chemical_list: D000900:Anti-Bacterial Agents