Table 11. DSM–IV Generalized Anxiety Disorder Item-by-Item Tests for Effects of Age, Study, and Age × Study on Factor Loadings.

Model	−2LnL	Δχ2	Δdf	AIC	Age effect	Study effect	Interaction effect
Age, study, and interaction on latent variance (baseline)	14,237.74			−14,572.26
Item free
Did your muscles often feel tense, sore or achy?	14,233.63	4.11	3	−14,570.37	−.03	−.16	.14
Did you often tire easily?	14,222.83	14.92**	3	−14,581.17	.57	.23	−.32
Were you so nervous you had trouble concentrating?	14,221.83	15.91**	3	−14,582.17	−.20	.08	.16
Did you often have trouble falling asleep?	14,235.36	2.38	3	−14,568.64	.13	.16	−.22
Were you often irritable or especially impatient?	14,212.33	25.40***	3	−14,591.66	−.35	−.15	.01
Did you feel restless or keyed up and on edge?	14,228.96	8.78*	3	−14,575.04	.24	−.14	.01

Note. N = 6,426. The most parsimonious model is shown in boldface. DSM–IV = Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition; AIC = Akaike information criterion.

^*p < .05.

^**p < .01.

^***p < .001.