title: Does quality of observational studies affect the results of a meta-analysis?: the case of cigarette smoking and pancreatic cancer.
abstract: OBJECTIVE
  A meta-analysis of observational studies on association between cigarette smoking and pancreatic cancer was performed to focus, particularly, on the role of the studies' quality in affecting meta-analysis results.
  
  
  METHODS
  A bibliographic search was carried out on PubMed and EMBASE databases until February 15, 2008. Key words were "pancreatic neoplasms," "pancreatic cancer," "smoking," "smoke," "cigarette," "case-control studies," and "cohort studies." Studies about cigarette smoking and pancreatic cancer were selected and assessed on quality. STATA 9.0 was used.
  
  
  RESULTS
  Six cohort studies and 24 case-control studies were selected, with median quality scores of 8 (range, 3) and 10 (range, 8), respectively. Pooled case-control studies' odds ratio (OR) and cohort studies' risk ratio were, respectively, 1.45 (95% confidence interval [CI], 1.33-1.57) and 1.78 (95% CI, 1.64-1.92). After stratifying for quality scoring, high-quality-scored case-control studies yielded an OR of 1.38 (95% CI, 1.27-1.49), whereas the others gave an OR of 1.52 (95% CI, 1.34-1.73). The results of meta-analysis for cohort studies showed a risk ratio of 1.74 (95% CI, 1.61-1.90) and of 2.10 (95% CI, 1.64-2.67), respectively, for high- and low-quality score studies.
  
  
  CONCLUSIONS
  There is evidence that cigarette smoking is an important risk factor for pancreatic cancer, but the estimate of the association greatly relies on the studies' quality.
authors.affiliation: Epidemiology and Biostatistics Unit, Institute of Hygiene, Catholic University of the Sacred Heart, Rome, Italy. giuseppe.latorre@rm.unicatt.it
authors.forename: Giuseppe
authors.initials: G
authors.lastname: La Torre
authors.forename: Chiara
authors.initials: C
authors.lastname: de Waure
authors.forename: Maria Lucia
authors.initials: ML
authors.lastname: Specchia
authors.forename: Nicola
authors.initials: N
authors.lastname: Nicolotti
authors.forename: Silvio
authors.initials: S
authors.lastname: Capizzi
authors.forename: Antonella
authors.initials: A
authors.lastname: Bilotta
authors.forename: Gennaro
authors.initials: G
authors.lastname: Clemente
authors.forename: Walter
authors.initials: W
authors.lastname: Ricciardi
mesh_terms: D016022:Case-Control Studies; D015331:Cohort Studies; D006801:Humans; D010190:Pancreatic Neoplasms / Q000453:epidemiology*; D012307:Risk Factors; D012907:Smoking / Q000453:epidemiology*