Table 2.

Final Model for Conventional Volumes^a

Variable	df	F Value	P Value
TS	1,275	4.96	.03
Age	1,275	2.45	.12
Sex	1,275	0.03	.86
Hemisphere	1,281	0.76	.38
OCD	1,275	0.09	.76
ADHD	1,275	0.20	.65
WBV	1,275	42.11	<.001
Age×hemisphere	1,282	5.55	.02

Abbreviations: ADHD, attention-deficit/hyperactivity disorder; df, degrees of freedom; OCD, obsessive-compulsive disorder; TS, Tourette syndrome; WBV, whole-brain volume.

The model was determined through a backward stepwise procedure for variable selection in which all main effects were forced into the model and the least significant higher-order terms were successively removed, with the constraint that the model was hierarchically well formulated at each step. The significant main effect of TS reflected larger overall thalamic volumes in the TS group. Additional significant terms included WBV, indicating the presence of scaling effects in the data (the larger the brain, the larger the thalamus), and the age×hemisphere interaction, indicating that the association of age with volume varied by hemisphere independent of tic status. The value for the mean volume of the right thalamus was larger than that for the left thalamus regardless of age; however, least squares means analysis suggested that the volume difference between hemispheres was significantly larger in adults (estimate, 268.92; SE, 48.15; t₁₁₈=5.59; P <.001) than children (estimate, 92.48; SE, 41.57; t₁₆₂=2.22; P=.03).