Table 2.

Differences in characteristics of the pregnant women between practices that were invited into the study versus practices that were not invited into the study, practices that accepted participation in the study versus practices that declined to participate among those invited and practices that recruited women into the study versus those that did not recruit women into the study among those which had agreed to participate.

		Practice invited								Practice not invited
		Practice accepted participation						Practice did not accept participation
		Practice recruiting				Practice not recruiting
		Women included		Women not included
		N = 1434	3.56%	N = 5364	13.31%	N = 2230	5.53%	N = 4344	10.78%	N = 26929	66.82%
Age			0.06 (0.01–0.22)		0.03 (0.01–0.19)		0.10 (0.02–0.32)		0.25 (0.13–0.41)
	<25 years	262	18.27	1141	21.27	447	20.04	728	16.76	3991	14.82
	26–30 years	540	37.66	1872	34.90	744	33.36	1628	37.48	9138	33.93
	31–35 years	416	29.01	1522	28.37	667	29.91	1272	29.28	8582	31.87
	>35 years	216	15.06	829	15.45	372	16.68	716	16.48	5218	19.38
Marital status			0.22 (0.09–0.44)		0.04 (0.00–0.20)		0.05 (0.00–0.20)		0.01 (0.00–0.04)
	Married	563	39.26	2258	42.10	914	40.99	1675	38.56	11092	41.19
	Cohabiting	675	47.07	2178	40.60	901	40.40	1860	42.82	11310	42.00
	Living alone	196	13.67	928	17.30	415	18.61	809	18.62	4527	16.81
Country of origin			3.01 (2.15–3.93)		0.20 (0.00–1.21)		0.04 (0.00–0.32)		0.08 (0.00–0.27)
	Denmark	1316	91.77	4051	75.52	1660	74.44	3292	75.78	20018	74.34
	Other	118	8.23	1313	24.48	570	25.56	1052	24.22	6911	25.66
Education level			1.41 (1.01–1.99)		0.09 (0.04–0.49)		0.43 (0.13–1.01)		0.58 (0.31–0.97)
	Primary school	171	11.92	931	17.36	399	17.89	610	14.04	3051	11.33
	Education = 3 years	583	40.66	1984	36.99	780	34.98	1317	30.32	7827	29.07
	Education = 4 or 5 years	436	30.40	1152	21.48	473	21.21	1059	24.38	6554	24.34
	Education > 5 years	208	14.50	676	12.60	333	14.93	821	18.90	6077	22.57
	Missing	36	2.51	621	11.58	245	10.99	537	12.36	3420	12.70
Occupation			1.10 (0.72–1.64)		0.01 (0.00–0.22)		0.03 (0.01–0.20)		0.07 (0.02–0.19)
	Employed	1125	78.45	3667	68.36	1548	69.42	3044	70.07	19486	72.36
	Unemployed	71	4.95	541	10.09	203	9.10	347	7.99	1853	6.88
	Student	171	11.92	576	10.74	245	10.99	515	11.86	3049	11.32
	Other	67	4.67	580	10.81	234	10.49	438	10.08	2541	9.44
Household income			0.90 (0.52–1.45)		0.07 (0.01–0.35)		0.10 (0.04–0.27)		0.40 (0.23–0.60)
	≤39,999 EUR	160	11.16	1129	21.05	479	21.48	915	21.06	4811	17.87
	40,000–79,999 EUR	468	32.64	1928	35.94	816	36.59	1505	34.65	8610	31.97
	80,000–119,000 EUR	565	39.40	1608	29.98	661	29.64	1230	28.31	7704	28.61
	≥120,000 EUR	241	16.81	690	12.86	274	12.29	691	15.91	5755	21.37
	Missing	0	0.00	9	0.17	0	0.00	3	0.07	49	0.18
Previously given birth			0.54 (0.08–1.39)		0.02 (0.00–0.11)		0.02 (0.00–0.09)		0.00 (0.00–0.01)
	Yes	723	50.42	2176	40.57	989	44.35	1941	44.68	11785	43,76
	No	711	49.58	3188	59.43	1241	55.65	2403	55.32	15144	56,24
GP consultations per year			0.11 (0.01–0.35)		0.02 (0.00–0.21)		0.04 (0.00–0.22)		0.01 (0.00–0.06)
	1–5 times	562	39.19	1984	36.99	860	38.57	1672	38.49	10393	38.59
	6–10 times	589	41.07	2092	39.00	838	37.58	1760	40.52	10684	39.67
	11 times or more	283	19.74	1288	24.01	532	23.86	912	20.99	5852	21.73
Use of medicine (ATC-group N)*			0.00 (0.00–0.07)		0.00 (0.00–0.11)		0.12 (0.01–0.36)		0.07 (0.02–0.16)
	No	1105	77.06	4155	77.46	1724	77.31	3501	80.59	21762	80.81
	Yes	329	22.94	1209	22.54	506	22.69	843	19.41	5167	19.19

Furthermore, it shows differences in characteristic of women who were included in the study versus women who were not included in the study from the practices that actively recruited women into the study. The boxes show the uncertainty coefficient in % (95% confidence interval in brackets), which quantifies the difference in distribution and thereby the relative strength of the selection effect in each step for each characteristic of the women. The uncertainty coefficient builds on Goodman and Kruskal’s classic review of association measures [18].