. Author manuscript; available in PMC: 2013 Mar 27.

Published in final edited form as: JAMA Psychiatry. 2013 Feb;70(2):235–242. doi: 10.1001/jamapsychiatry.2013.276

Table 2.

Results From Full Cox Proportional Hazards Model for the Risk for Drug Abuse as a Function of Whether the Drug Abuse–Affected Sibling Was Older vs Younger

Variable

Hazard Ratio (95% CI)

χ_{1}^{2}

P Value

Older vs younger sibling

1.42 (1.31–1.54)

70.2

<.001

Age differences

0.88 (0.87–0.90)

128.9

<.001

Age differences²

1.01 (1.00–1.01)

38.8

<.001

Sibling pair by sex

Male-male

1.0 [Reference]

Male-female

0.35 (0.33–0.37)

1341.5

<.001

Female-female

0.52 (0.47–0.58)

135.1

<.001

Female-male

0.73 (0.66–0.80)

42.4

<.001

Birth cohort

1950–1959

1.0 [Reference]

1960–1969

1.65 (1.54–1.77)

195.4

<.001

1970–1979

2.95 (2.75–3.17)

885.2

<.001

1980–1993

3.90 (3.63–4.20)

1390.1

<.001

Interaction terms

Older sibling, age differences

1.03 (1.02–1.04)

15.6

<.001

Older sibling, female-female

0.85 (0.74–0.98)

5.0

.03

Older sibling, female-male

0.87 (0.77–0.98)

5.1

.02