Non-linear dose-response relationship between pack-years of cigarettes smoking and relative risk of RA. Relative risk (solid line) and 95% confidence interval (long dashed lines) from the restricted cubic splines model. The short dashed line represents the RR from the linear model. Estimates reported in the table are based on median value of each category.
Di Giuseppe et al. Arthritis Research & Therapy 2014 16:R61 doi:10.1186/ar4498