Intended for healthcare professionals


Reanalysis of epidemiological evidence on lung cancer and passive smoking

BMJ 2000; 320 doi: (Published 12 February 2000) Cite this as: BMJ 2000;320:417
  1. J B Copas, professor (jbc{at},
  2. J Q Shi, research fellow
  1. Department of Statistics, University of Warwick, Coventry CV4 7AL
  1. Correspondence to: J B Copas
  • Accepted 1 November 1999


Objective: To assess the epidemiological evidence for an increase in the risk of lung cancer resulting from exposure to environmental tobacco smoke.

Design: Reanalysis of 37 published epidemiological studies previously included in a meta-analysis allowing for the possibility of publication bias.

Main outcome measure: Relative risk of lung cancer among female lifelong non-smokers, according to whether her partner was a current smoker or a lifelong non-smoker.

Results: If it is assumed that all studies that have ever been carried out are included, or that those selected for review are truly representative of all such studies, then the estimated excess risk of lung cancer is 24%, as previously reported (95% confidence interval 13% to 36%, P<0.001). However, a significant correlation between study outcome and study size suggests the presence of publication bias. Adjustment for such bias implies that the risk has been overestimated. For example, if only 60% of studies have been included, the estimate of excess risk falls from 24% to 15%.

Conclusion: A modest degree of publication bias leads to a substantial reduction in the relative risk and to a weaker level of significance, suggesting that the published estimate of the increased risk of lung cancer associated with environmental tobacco smoke needs to be interpreted with caution.

Key messages

  • A systematic review of epidemiological studies on passive smoking estimated the increased risk of lung cancer as 24%

  • There is clear evidence of publication bias in these studies

  • Reanalysis of the data allowing for the possibility of publication bias substantially lowers the estimate of relative risk


  • Funding Economic and Social Research Council.

  • Competing interest None declared.

  • Accepted 1 November 1999
View Full Text