Response rates and selection problems, with emphasis on mental health variables and DNA sampling, in large population-based, cross-sectional and longitudinal studies of adolescents in Norway

Table 3 Baseline socio-demographic characteristics in 2001, prevalence and relative risk (RR) of lost to follow-up in 2004 in Oslo and Hedmark combined

Variables collected	Base-line	Lost to follow-up		Relative risk of lost to follow-up
In 2001	N	n	%	RR_crude ¹	95% CI		RR_adj ^1,2	95% CI
Gender ³
Boys	2894	1455	50	Ref			Ref
Girls	2835	958	34	0.67	0.63	0.72	0.72	0.67	0.77
Ethnicity ⁴
Western	4768	1948	41	Ref			Ref
Non western	896	428	48	1.17	1.08	1.26	1.31	1.21	1.43
Invitation group
Oslo school	2493	388	16	Ref
Oslo post	1145	761	66	4.27	3.86	4.72
Hedmark post	1710	883	52	3.32	3.00	3.68
Education plans
University/higher	2847	911	32	Ref			Ref
Other	1940	1119	58	1.80	1.69	1.93	1.23	1.15	1.33
Not decided yet	859	336	39	1.22	1.11	1.35	1.07	0.96	1.18
Family economic situation
Bad	184	108	59	Ref			Ref
Average	1824	847	46	0.79	0.69	0.90	0.86	0.75	0.99
Good	3014	1155	38	0.65	0.57	0.74	0.82	0.71	0.94
Very good	620	266	43	0.73	0.63	0.85	0.95	0.81	1.12
Education father
Lower secondary	786	381	48	Ref			Ref
Upper secondary	2470	1082	44	0.90	0.83	0.98	0.97	0.89	1.05
University/College	1837	471	26	0.53	0.48	0.59	0.79	0.71	0.87
Income father
Low	1190	585	49	Ref			Ref
Medium	2654	1011	38	0.77	0.72	0.84	0.84	0.78	0.90
High	1161	304	26	0.53	0.48	0.60	0.80	0.72	0.88
Parents' marital status
Married	3891	1518	39	Ref			Ref
Not married	1806	876	49	1.24	1.17	1.32	1.13	1.06	1.21

¹When substituting "lost to follow-up" with "failure to provide DNA" as dependent variable, the RRs were similar;
²Adjusted for invitation group;
³Gender does not sum up to 5750 as shown in Figure 1 and Table 1 due to "missing information on gender" on 21 individuals;
⁴Based on self-reported native country of mother and father.

ISSN: 1471-2458