| Factor | RR (95% CI) | p-value |
| Smoking status | ||
| Current versus former | 0.9 (0.8–0.9) | <0.001 |
| Age yrs | 0.064 | |
| 55–64 versus <55 | 1.1 (1.0–1.3) | 0.012 |
| 65–74 versus <55 | 1.1 (1.0–1.3) | 0.020 |
| ≥75 versus <55 | 1.2 (1.0–1.3) | 0.023 |
| Post-bronchodilator FEV1 % pred | <0.001 | |
| <30% versus ≥50% | 1.9 (1.7–2.1) | <0.001 |
| 30 to <50% versus ≥50% | 1.4 (1.3–1.5) | <0.001 |
| Sex | ||
| Male versus female | 0.7 (0.7–0.8) | <0.001 |
| COPD exacerbations in the previous year | <0.001 | |
| 1 versus 0 | 1.5 (1.4–1.6) | <0.001 |
| ≥2 versus 0 | 1.9 (1.8–2.1) | <0.001 |
| BMI kg·m−2 | <0.001 | |
| <20 versus 20 to <25 | 1.1 (1.0–1.2) | 0.241 |
| 25 to <29 versus 20 to <25 | 1.0 (0.9–1.0) | 0.262 |
| ≥29 versus 20 to <25 | 0.9 (0.8–0.9) | <0.001 |
| MRC dyspnoea grade | <0.001 | |
| 3 versus 1+2 | 1.1 (1.1–1.2) | <0.001 |
| 4+5 versus 1+2 | 1.3 (1.2–1.4) | <0.001 |
| Region | <0.001 | |
| North versus south | 0.8 (0.7–0.9) | <0.001 |
| Tropics versus south | 0.9 (0.8–1.0) | 0.045 |
Rate ratios (RR) with 95% confidence intervals were calculated for each covariate with all other covariates included in the model. Multivariate negative binomial model (each variable adjusted for all the others). FEV1: forced expiratory volume in 1 s; % pred: % predicted; COPD: chronic obstructive pulmonary disease; BMI: body mass index; MRC: Medical Research Council.