Table 3.

Chi-square tests of interaction between group status (patients, sibling, or control) and childhood trauma or childhood urbanicity in the model predicting educational difference

	χ 2a	Df b	p
Childhood trauma	2.37	2	0.31
Childhood trauma—dummyc	3.25	4	0.52
Urbanicity	1.67	2	0.43
Urbanicity—dummyd	7.50	8	0.48

Childhood trauma and childhood urbanicity are both categorical variables with n categories, entered both as a continuous variable and as n − 1 dummy variables

^aChi square

^bDegrees of freedom

^cChildhood trauma scores divided in tertiles (low, medium, high)

^dFive levels of urbanicity/population density 1 <500 inhabitants/km²; 2 500–1000 inhabitants/km²; 3 inhabitants 1000–1500/km²; 4 inhabitants 1500–2500/km²; 5 2500+/km