PURPOSE: To systematically review observational studies on medical interventions to determine the quality of reporting of confounding.
METHODS: Articles on observational studies on medical interventions in five general medical journals and five epidemiological journals published between January 2004 and April 2007 were systematically reviewed. All relevant items pertaining to confounding bias were scored for each article. The overall quality of reporting was determined with an 8-point score.
RESULTS: The MEDLINE search resulted in 2993 publications, and 174 (5.8%) articles were included in the analysis. In the majority of studies (>98%), the potential for confounding bias was reported. Details on the selection and inclusion of observed confounders were reported in 10% and 51%, respectively. The potential for unobserved confounding was reported in 60%, and 9% commented on the potential effect of such remaining confounding. The quality of reporting of confounding score was mediocre (a median score of 4 points; interquartile range 3 to 5), and scores were similar in all years.
CONCLUSION: The quality of reporting of confounding in articles on observational medical intervention studies was poor. However, the STROBE statement for reporting of observational studies may considerably impact the reporting of such studies.