Table 3.

Stratified main Cox model risk ratios (relative risks) for incidence rate of chronic obstructive pulmonary disease (COPD), for smoking, occupation, and cooking Xuanwei, 1976-92

Independent variables	Relative risk (95% CI)	P value
Men only
Amount of smoking (<20*†‡ pack years v all others):
20-29‡	1.16 (0.93 to 1.44)	0.007
30-39‡	0.97 (0.73 to 1.29)
≥40‡	1.45 (1.14 to 1.86)
Cooked food <20 years*‡§v ≥20 years	0.95 (0.72 to 1.27)	0.748
Never a coal miner*v ever a coal miner	1.22 (0.93 to 1.61)	0.160
Women only
Amount of cooking (cooked food <20 years*‡§v all others):
20-29‡	1.07 (0.74 to 1.55)	0.602
30-39‡	1.09 (0.73 to 1.62)
≥40‡	1.31 (0.83 to 2.07)

^*Reference category (relative risk=1).

^†Includes non-smokers.

^‡Time dependent number of pack years of men's smoking, duration of men's cooking, and duration of women's cooking, from age at starting to smoke or cook to age at each successive risk set comparison.

^§Includes people who did not cook food.