title: Systematic reviews synthesized evidence without consistent quality assessment of primary studies examining epidemiology of chronic diseases.
abstract: OBJECTIVE
  To evaluate how systematic reviews assess the quality of primary studies of incidence, prevalence, or risk factors for chronic diseases.
  
  
  METHODS
  We searched several databases, identified 145 systematic reviews, and evaluated methods of quality assessment and quantitative synthesis of evidence by external or internal validity or overall quality of primary studies.
  
  
  RESULTS
  Of 145 reviews, 54 (37%) reported a planned quality assessment of primary studies with checklists or scales and 26 (18%) reported evaluation of some selected quality criteria. Thirty-nine percent of reviews judged appropriateness of sampling and proper controls for confounding factors in primary studies. Twelve percent synthesized evidence by overall quality, 17% by design, 42% by criteria of internal validity, and 24% by external validity of primary studies. Masking of quality assessment was conducted on 2.1% of reviews and 4% tested interobserver agreement for quality assessment.
  
  
  CONCLUSIONS
  Evaluation of internal and external validity of primary studies is uncommon in systematic reviews of studies of incidence, prevalence, or risk factors for chronic diseases. Inconsistent quality assessment practices reflect the absence of uniformly accepted standards and tools to examine the quality of observational nontherapeutic studies.
authors.affiliation: Minnesota Evidence-based Practice Center, University of Minnesota School of Public Health, Minneapolis, MN 55455, USA. shaml005@umn.edu
authors.forename: Tatyana
authors.initials: T
authors.lastname: Shamliyan
authors.forename: Robert L
authors.initials: RL
authors.lastname: Kane
authors.forename: Stacy
authors.initials: S
authors.lastname: Jansen
mesh_terms: D002908:Chronic Disease / Q000453:epidemiology*; D019317:Evidence-Based Medicine / Q000592:standards; D006302:Health Services Research / Q000592:standards*; D006801:Humans; D015994:Incidence; D015995:Prevalence; D011785:Quality Assurance, Health Care / Q000592:standards; D015203:Reproducibility of Results; D012107:Research Design / Q000592:standards*; D058028:Research Report / Q000592:standards*; D012196:Review Literature as Topic; D012307:Risk Factors; D015983:Selection Bias; D014481:United States / Q000453:epidemiology