Table 1 Comparison of baseline characteristics for those with and without spirometry at ~17.5 years in Hong Kong’s “Children of 1997” birth cohort.

	Participants with spirometric data (N = 3033)		Participants without spirometric data (N = 5265)		Cohen’s w
	N	%	N	%	Cohen’s w
Sex					0.03
Female	1475	49.0	2426	46.4
Male	1522	51.0	2803	53.6
Sex and gestational age adjusted birth weight Z-score					0.02
≤−2 SD	48	1.6	85	1.7
−2 SD–1SD	390	13.1	699	13.7
−1 SD −+1 SD	2100	70.4	3512	69.1
+1 SD −+2 SD	361	12.1	629	12.4
>2 SD	83	2.8	159	3.1
Birth Order					<0.01
1	1403	47.5	2391	47.7
2	1225	41.5	2065	41.2
3 or above	323	10.9	556	11.1
Preterm birth					0.03
Preterm	136	4.5	297	5.7
Full-term	2883	95.5	4878	94.2
Maternal age at delivery, years					0.07
≤24	294	9.7	714	14.0
25–29	944	31.2	1568	30.9
30–34	1176	38.9	1908	37.7
≥35	607	20.1	873	17.2
Mother’s birthplace					0.04
Hong Kong	1769	58.7	3018	62.6
Mainland China	1247	41.3	1805	37.4
Maternal smoking					0.04
No	2839	96.2	4722	94.2
Yes	112	3.8	291	5.8
Household income					0.04
1^st quintile	496	18.2	955	21.2
2^nd quantile	552	20.2	932	20.7
3^rd quantile	545	20.0	884	19.6
4^th quantile	552	20.3	867	19.3
5^th quantile	575	21.1	865	19.2
Highest parental education					0.04
Grade 9 or below	861	28.5	1606	31.6
Grade 10–11	1296	42.9	2151	42.4
Grade 12 or above	863	28.6	1320	26.0
Highest parental occupation					0.05
I (professional)	711	26.8	1003	22.7
II (managerial)	392	14.8	730	16.5
IIINM (non-manual skilled)	762	28.8	1284	29.1
IIIM (manual skilled)	444	16.8	771	17.5
VI (semiskilled)	256	9.7	470	10.7
V (unskilled)	85	3.2	154	3.5

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