Table 2. Traditional (least squares) and quantile regression analyses of spirometric traits between related individuals.^a.

	Least-squares regression analysis			Quantile regression analysis
	Correlation	Least-squares regression slope		Increase in slope per 1% increase in the offspring or sibling distribution		Difference in slope between the 90th and 10th percentiles
		Slope ± SE	Pb	Slope ± SE	Pc	Difference ± SE	Pd
FEV1/FVC
Spouse	0.04	0.047 ± 0.018	0.007	−0.0005 ± 0.0005	0.37	−0.008 ± 0.077	0.91
Offspring-parent	0.17	0.134 ± 0.010	<10−15	−0.0009 ± 0.0003	0.003	−0.083 ± 0.032	0.01
Offspring-midparent	0.24	0.269 ± 0.018	<10−15	−0.0021 ± 0.0005	4.1 × 10−5	−0.192 ± 0.059	0.001
Full-sibling	0.27	0.266 ± 0.017	<10−15	−0.0022 ± 0.0005	2.1 × 10−6	−0.200 ± 0.040	5.6 × 10−7
FVC
Spouse	0.08	0.109 ± 0.033	0.001	0.0008 ± 0.0009	0.41	0.049 ± 0.089	0.58
Offspring-parent	0.26	0.263 ± 0.013	<10−15	−0.0002 ± 0.0003	0.43	−0.031 ± 0.027	0.24
Offspring-midparent	0.32	0.463 ± 0.022	<10−15	−0.0007 ± 0.0006	0.23	−0.050 ± 0.059	0.4
Full-sibling	0.32	0.321 ± 0.017	<10−15	0.0001 ± 0.0004	0.76	−0.001 ± 0.039	0.99
FEV1
Spouse	0.08	0.118 ± 0.034	0.0005	−0.0010 ± 0.0008	0.23	−0.073 ± 0.090	0.40
Offspring-parent	0.23	0.227 ± 0.012	<10−15	−0.0003 ± 0.0003	0.29	−0.026 ± 0.034	0.44
Offspring-midparent	0.30	0.407 ± 0.022	<10−15	−0.0007 ± 0.0006	0.22	−0.009 ± 0.058	0.88
Full-sibling	0.28	0.282 ± 0.017	<10−15	−0.0006 ± 0.0004	0.17	−0.070 ± 0.045	0.12
PEF
Spouse	0.05	0.078 ± 0.045	0.08	0.0001 ± 0.0012	0.92	−0.047 ± 0.111	0.67
Offspring-parent	0.19	0.180 ± 0.014	<10−15	0.0003 ± 0.0003	0.32	0.017 ± 0.034	0.61
Offspring-midparent	0.25	0.336 ± 0.026	<10−15	0.0006 ± 0.0006	0.34	0.052 ± 0.055	0.34
Full-sibling	0.23	0.228 ± 0.017	<10−15	0.0003 ± 0.0004	0.46	0.063 ± 0.046	0.17
MMEF
Spouse	0.04	0.049 ± 0.038	0.20	−0.0012 ± 0.0009	0.20	−0.106 ± 0.097	0.28
Offspring-parent	0.25	0.285 ± 0.015	<10−15	0.0013 ± 0.0004	0.0004	0.108 ± 0.037	0.003
Offspring-midparent	0.35	0.550 ± 0.028	<10−15	0.0014 ± 0.0008	0.06	0.137 ± 0.071	0.05
Full-sibling	0.30	0.296 ± 0.017	<10−15	0.0009 ± 0.0005	0.04	0.049 ± 0.049	0.32
FEF25%
Spouse	0.07	0.101 ± 0.043	0.02	−0.0007 ± 0.0011	0.52	−0.097 ± 0.105	0.36
Offspring-parent	0.21	0.200 ± 0.013	<10−15	−0.0002 ± 0.0003	0.45	−0.024 ± 0.032	0.47
Offspring-midparent	0.28	0.367 ± 0.024	<10−15	−0.0005 ± 0.0006	0.35	−0.092 ± 0.060	0.13
Full-sibling	0.24	0.241 ± 0.017	<10−15	−0.0007 ± 0.0004	0.12	−0.032 ± 0.035	0.36
FEF50%
Spouse	0.06	0.077 ± 0.037	0.04	−0.0007 ± 0.0009	0.41	−0.124 ± 0.080	0.12
Offspring-parent	0.25	0.253 ± 0.013	<10−15	0.0011 ± 0.0003	0.002	0.079 ± 0.037	0.03
Offspring-midparent	0.37	0.514 ± 0.024	<10−15	0.0010 ± 0.0006	0.10	0.060 ± 0.060	0.32
Full-sibling	0.28	0.284 ± 0.017	<10−15	0.0008 ± 0.0004	0.05	0.055 ± 0.044	0.21
FEF75%
Spouse	0.01	0.015 ± 0.038	0.70	−0.0012 ± 0.0011	0.27	−0.114 ± 0.097	0.24
Offspring-parent	0.20	0.249 ± 0.017	<10−15	0.0021 ± 0.0004	1.8 × 10−6	0.153 ± 0.048	0.002
Offspring-midparent	0.28	0.507 ± 0.033	<10−15	0.0035 ± 0.0009	0.0001	0.228 ± 0.090	0.01
Full-sibling	0.27	0.269 ± 0.017	<10−15	0.0019 ± 0.0005	0.0002	0.147 ± 0.052	0.005

Notes.

^aSee figure legends for sample sizes.

^bSignificance determined from standard least-squares regression giving each offspring equal weight.

^cSignificance determined from linear contrast from the variance–covariance matrix determined from 1000 bootstrap samples.

^dSignificance determined from difference in coefficients between the 10th and 90th quantiles and their associated standard errors (±SE) and covariance determined from 1,000 bootstrap samples. Significance levels are those reported by Stata, but it is important not to interpret significance levels much less than P < 0.001 literally but rather as indicators of strong significance.