Table 4.
Absolute rates and relative risks of all-cause mortality among Aboriginal participants in the 45 and Up Study, by smoking intensity for current-smokers, and by age at cessation for past-smokers, relative to never-smokers
| Deaths | P-years | Crude rate | RR1 (95% CI) | RR2 (95% CI) | ||
|---|---|---|---|---|---|---|
| Smoking intensity, in current-smokers (cigarettes/day)a | Never-smoker | 41 | 6214 | 6.60 | 1 (ref) | 1 (ref) |
| N = 870 | 1–14 | 11 | 810 | 13.58 | 2.75 (1.38–5.47) | 2.88 (1.43–5.80) |
| 15–24 | 22 | 1410 | 15.61 | 4.81 (2.76–8.38) | 4.55 (2.58–8.02) | |
| ≥25 | 12 | 757 | 15.85 | 4.47 (2.25–8.86) | 4.29 (2.15–8.57) | |
| Age at cessation (years), in past-smokers | Never-smoker | 41 | 6214 | 6.60 | 1 (ref) | 1 (ref) |
| N = 1283 | Quit at age ≤44 | 21 | 2902 | 7.24 | 1.38 (0.80–2.38) | 1.48 (0.85–2.57) |
| Quit at age 45–54 | 21 | 1424 | 14.74 | 2.28 (1.33–3.90) | 2.21 (1.29–3.80) | |
| Current-smoker | 48 | 3078 | 15.59 | 4.03 (2.60–6.25) | 3.98 (2.56–6.19) | |
Participants with missing values for smoking intensity or age at smoking cessation were excluded from regression using those variables. Past-smokers who quit smoking at age ≥55 years are excluded from regression using age at cessation. We could not reliably estimate the relation of later smoking cessation (≥55 years) to mortality, given that older smokers are likely to have quit due to illness. However, the vast majority of past-smokers in this cohort quit before age 55 years. The Cox regression models for smoking intensity RR1 and RR2 violated the proportional hazard assumption for the main exposure using the P-value threshold of 0.05. As age is used for the underlying time variable, violations of proportional hazards assumption are likely to be due to interaction with age.
RR1: adjusted for age as the underlying time variable and sex; RR2: additionally adjusted for education and remoteness. Rates are presented per 1000 person-years.
P-years, person-years; RR, relative risk; 95% CI, 95% confidence interval.
P-trend for RR2 <0.01 (only tested for smoking intensity). To test for a trend in the relationship between smoking intensity and mortality, the fully adjusted model was re-run with smoking intensity as a continuous variable, with each category recoded to the median smoking intensity within that category.