Z allele | p Value | Z×S interaction^{*} | p Value | Pack-years^{†} | p Value | ||

Model for FEV_{1} (% predicted) | Linear | −1.87 | 0.62 | 0.16 | 0.27 | −0.33 | <0.001 |

Piecewise linear | −5.59 | 0.14 | 0.35 | 0.03 | −0.38 | <0.001 | |

Model for COPD status | Linear | 2.78 | 0.07 | 0.97 | 0.09 | 1.05 | <0.001 |

Piecewise linear | 4.87 | 0.01 | 0.94 | 0.01 | 1.08 | <0.001 |

Regression models adjusted for age, sex and smoking (using either a linear or piecewise linear form of the pack-years variable). The FEV

_{1}model includes a sample weight adjustment to reflect oversampling of COPD cases.Results reported as β coefficients for FEV

_{1}model, ORs for COPD model and their respective p values.* The Z×S interaction term is of the form (number of copies of Z allele × pack-years) for the linear smoking adjustment and (number of copies of Z allele × first 40 pack-years) for the linear and piecewise linear adjustments, respectively.

† Smoking adjustment for the linear pack-years model is done by including a numerical term for total pack-years of smoking. For the piecewise linear model, the smoking main effect is represented by two variables representing the first 40 pack-years of smoking and subsequent exposure. The reported beta coefficient is for the term representing the first 40 pack-years of smoking.

COPD, chronic obstructive pulmonary disease; FEV

_{1}, forced expiratory volume in 1 s.