A case-control study examining the association between pesticide exposure and lung cancer in agricultural workers reports an unadjusted odds ratio of 3.2 (95% CI: 2.1–4.8). After adjustment for smoking using multivariate logistic regression, the adjusted OR becomes 2.8 (95% CI: 1.9–4.1). The researchers suspect smoking may be a confounder. Which investigation would be most appropriate to formally assess whether smoking meets the statistical criteria for confounding?