Table 1.

Baseline characteristics of the study population according to the level of education in a large registry-based sibling study in Norway.

	Primary	Secondary	Undergraduate	Graduate
Cases
No. of participants	1003	2186	1050	255
Year of birth, mean	1958.3	1959.2	1961.8	1961.8
No. of siblings, mean	2.2	2.0	2.0	1.9
First born, %	31.4	37.9	39.5	41.2
Female, %	64.7	63.7	75.2	58.0
Born in North tier,a %	14.3	12.6	14.2	11.7
Parents with higher education,b %	4.6	11.0	29.0	50.8
Controls
No. of participants	2104	4431	2063	595
Year of birth, mean	1958.6	1959.5	1961.5	1961.3
No. of siblings, mean	3.0	2.8	2.6	2.5
First born, %	27.7	31.3	32.7	35.0
Female, %	47.6	44.7	59.6	36.0
Born in North tier,a %	15.0	14.3	17.6	12.5
Parents with higher education,b %	5.0	9.7	26.4	46.6

^aBorn in one of the three most Northern counties in Norway (Nordland, Troms and Finmark).

^bDefined as at least one parent with undergraduate or graduate level of education.