Mediation effects of 5-HT on the relation between pack-years smoked and COPD.

Since the level of 5-HT was not normally distributed, a log-transformation was performed in the linear regression model. All analyses were adjusted for age.SE = standard error.a,b,c,eRegression analyses were performed to calculate parameter estimates.dThe estimate is the product of the estimates of b and c. After the estimate and SE of the product were calculated, the Sobel test was performed to assess the significance of the indirect effect.