Forest plot of relative risk estimates for rheumatoid arthritis risk associated with pack-years of cigarette smoking (highest vs. lowest category). CI indicates confidence intervals; RR indicates relative risk. The size of each square is proportional to the study’s weight (inverse variance). *Test for heterogeneity: cohort studies, P = 0.96, I2 = 0.0%; case-control studies, P = 0.19, I2 = 28.7%, overall studies, P = 0.32, I2 = 12.7%.
Di Giuseppe et al. Arthritis Research & Therapy 2014 16:R61 doi:10.1186/ar4498