BMJ  2003;327:501 (30 August), doi:10.1136/bmj.327.7413.501-a

Letter

Passive smoking

Wider evidence needs to be interpreted

The first 150 words of the full text of this article appear below.

Editor—Enstrom and Kabat's analysis has several omissions.¹ First they accept that most epidemiological studies have found positive but not statistically significant relationships between environmental tobacco smoke, coronary heart disease, and lung cancer, but then argue against meta-analysis to establish a causal relation. This is precisely where systematic reviews, and sometimes meta-analysis, show considerable benefit by increasing power. Enstrom and Kabat say that publication bias may explain positive results in reviews; however, larger cohort studies, unlike small trials and reports, are more likely to be published, regardless of results.² They do not explain heterogeneity between their findings and others, simply arguing that their cohort is large, and has more strengths. In fact, large prospective cohort studies like this may have greater losses to follow up, or more misclassification, over time.³

Misclassification, mentioned by the authors, may explain the apparent lack of association. Furthermore, the relative risks reported for active smoking and . . . [Full text of this article]

International Health Research Group, Liverpool School of Tropical Medicine, Liverpool L3 5QA juliac@liverpool.ac.uk

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