Table 1

 Stepwise exclusion of subjects with respiratory diseases, symptoms and smoking history: mean (SD) FEV1, FVC and FEV1/FVC at each step

nFEV1FVCFEV1/FVC
FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity.
*p⩽0.05 between analysed group and those excluded since previous analysis.
†Lung function estimates were standardised for the distribution of sex, age and height in the parent population. Estimates not marked with † are non-standardised.
Parent population (A)23583.22 (0.94)4.09 (1.14)0.785 (0.067)
Subjects without cardinal symptoms (B), standardised†11843.35 (0.42)4.21 (0.53)0.796 (0.096)
Stepwise exclusion from population type A to type B
    Not obstructive lung diseases in 199621883.28 (0.91)*4.15 (1.12)*0.790 (0.059)*
    Not breathlessness grade 421483.29 (0.91)*4.16 (1.12)*0.790 (0.059)*
    Not breathlessness grade 320993.31 (0.90)*4.18 (1.11)*0.790 (0.058)*
    Not breathlessness grade 218863.36 (0.88)*4.25 (1.09)*0.791 (0.057)*
    Not morning cough14983.41 (0.89)*4.29 (1.11)*0.795 (0.054)*
    Not breathlessness grade 113773.44 (0.88)*4.33 (1.10)*0.796 (0.054)
    Not wheeze12053.43 (0.88)4.31 (1.09)*0.797 (0.053)*
    Not chronic cough11843.44 (0.88)*4.32 (1.09)*0.797 (0.053)
Never-smokers without cardinal symptoms (C), standardised†5393.39 (0.42)4.25 (0.53)0.799 (0.096)
Stepwise exclusion from population type B to type C
    Never-smokers5393.43 (0.94)4.28 (1.18)0.801 (0.051)*
Never-smokers without any symptoms (D), standardised†3313.40 (0.42)4.25 (0.53)0.799 (0.096)
    Stepwise exclusion from population type C to type D
    Not phlegm5013.43 (0.94)4.29 (1.17)0.801 (0.051)
    Not attacks of breathlessness4843.43 (0.93)4.30 (1.17)0.801 (0.052)
    Not day cough4773.44 (0.93)4.30 (1.16)0.801 (0.051)
    Not infancy lung disease4713.44 (0.93)4.30 (1.16)0.801 (0.051)
    Not cough with cold3353.40 (0.95)4.25 (1.18)0.800 (0.053)
    Not woken by breathlessness3323.40 (0.95)4.26 (1.19)0.800 (0.053)
    Not breathlessness and wheeze3313.40 (0.95)4.26 (1.19)0.799 (0.053)