Table 4.

The association between smoking and lung function, with the regression coefficients (β) and p values for each pack-year's impact on FEV₁ (ml)^* and FEV₁% predicted^†

		FEV1 (ml)			FEV1 (%)
	n‡	β (95% CI)	p Value	p (interaction)	β (95% CI)	p Value	p (interaction)
Subjects with COPD
All§	948	−3.6 (−5.6 to −1.7)	0.000		−0.11 (−0.17 to −0.04)	0.001
Male¶	583	−5.1 (−7.4 to −2.7)	0.000		−0.15 (−0.22 to −0.08)	0.000
Female¶	365	−0.1 (−3.7 to 3.5)	0.952	0.025	0.002 (−0.11 to 0.12)	0.967	0.027
Subjects without COPD
All§	954	−3.1 (−4.6 to −1.6)	0.000		−0.09 (−0.13 to −0.05)	0.000
Male¶	478	−3.2 (−5.1 to −1.2)	0.001		−0.06 (−0.12 to −0.01)	0.027
Female¶	476	−3.0 (−5.4 to −0.7)	0.011	0.940	−0.13 (−0.20 to −0.06)	0.000	0.124

p (interaction) is the p value for the interaction term gender×pack-years.

^*Adjusted for sex, age, height, pack-years, self-reported asthma and educational level.

^†Adjusted for sex, pack-years, self-reported asthma and educational level.

^‡Educational level was missing for five female subjects with COPD and one of the male subjects without COPD; asthma history was missing for one of the female subjects with COPD.

^§Models without an interaction term.

^¶Models including a gender×pack-years interaction term; gender-specific estimates obtained by the lincom command in Stata.

COPD, chronic obstructive pulmonary disease; FEV₁, forced expiratory volume in 1 s.