Use of propensity score matching in evaluating smokeless tobacco as a gateway to smoking

The contentious debate over the promotion of Swedish snus, a form of moist snuff, as an alternative to cigarettes has often focused on the product's potential as a gateway to smoking. Findings from prospective studies in the United States have suggested that smokeless tobacco (SLT) is a significant predictor of smoking onset, before and after adjustment for baseline covariates. Use of conventional regression methods in these studies may have resulted in biased parameter estimates, arising from imbalanced covariate distributions in the users and nonusers of SLT. An alternative approach, which has been used widely in the econometric literature, matches exposure or treatment levels on the basis of the propensity score distribution.

Using this approach, we matched current SLT users from the National Longitudinal Study of Adolescent Health with nonusers (496 pairs) and followed them from adolescence into young adulthood for determination of smoking status.

Prior to matching, the unadjusted risk of becoming a daily smoker was significantly greater for the SLT users compared with nonusers (n = 10,820; range of relative risk = 1.3-2.0, p < .001). However, after pairing individuals on propensity score, we found no evidence for an increased risk of smoking among the SLT users.

Baseline differences in the risk factors for smoking likely account for the association between the two tobacco products.