Table 3.

Typical and Atypical Sequencing Pattern Results from Baseline Multilevel Proportional Hazard Regression Models

Typical Sequencing		Atypical Sequencing
	HR (95% CI)		HR (95% CI)
Tobacco Initiation → Alcohol Initiation (n = 2,558)		Alcohol Initiation → Tobacco Initiation (n = 1,722)
Age	1.00 (.98, 1.02)	Age	1.00 (.97, 1.02)
Sex	.97 (.89, 1.06)	Sex	1.05 (.93, 1.19)
WTP age of tobacco initiation	.82 (.80, .84)	WTP age of alcohol initiation	.74 (.71, .77)
BTP age of tobacco initiation	.34 (.25, .46)	BTP age of alcohol initiation	.87 (.85, .90)
Tobacco Initiation → Cannabis Initiation (n = 2,224)		Cannabis Initiation → Tobacco Initiation (n = 338)
Age	.99 (.97, 1.01)	Age	.98 (.92, 1.04)
Sex	1.00 (.91, 1.10)	Sex	.88 (.66, 1.17)
WTP age of tobacco initiation	.93 (.90, .95)	WTP age of cannabis initiation	.07 (.02, .28)
BTP age of tobacco initiation	.52 (.40, .68)	BTP age of cannabis initiation	.01 (.004, .03)
Alcohol Initiation → Cannabis Initiation (n = 2,410)		Cannabis Initiation → Alcohol Initiation (n = 552)
Age	.99 (.98, 1.01)	Age	1.04 (.99, 1.08)
Sex	.96 (.88, 1.06)	Sex	.004 (.00, .96)
WTP age of alcohol initiation	.89 (.86, .91)	WTP age of cannabis initiation	.58 (.53, .63)
BTP age of alcohol initiation	.55 (.39, .77)	BTP age of cannabis initiation	.002 (.001, .006)

Notes. HR = hazard ratio, CI = confidence interval, WTP = within-twin-pair, BTP = between-twin-pair. Each model included individuals from twin pairs in which at least one twin followed the designated initiation sequence and the co-twin had used the first substance. Individuals who reported using both substances at the same age were included in models for both typical and atypical sequencing. When a more conservative alpha level of .01 was used for calculation of confidence limits, the only effect that did not remain significant was the between-twin-pair effect for the age of cannabis initiation predicting the age of alcohol initiation.