Table 3.

Comparison of Two Zero-Inflated Negative Binomial Regression Models of Factors Associated with the Number of Primary Care Visits among Young Adults on the Autism Spectrum, 2010

	Regression 1+ sample a				Regression 2+ sample b
	IRR	SE	Lower 95% CI	Upper 95% CI	IRR	SE	Lower 95% CI	Upper 95% CI
Primary care visits
ASD and ID	0.986	0.089	0.826	1.178	0.926	0.100	0.750	1.144
Age 22–27	1.046	0.110	0.851	1.284	1.003	0.127	0.782	1.286
Minority	1.027	0.100	0.849	1.242	0.993	0.114	0.792	1.244
Female	1.136	0.116	0.930	1.388	1.106	0.139	0.864	1.416
Dual eligibility: Medicaid 1–12 months	0.903	0.122	0.693	1.176	0.937	0.149	0.687	1.278
Psychiatric primary claim	0.914	0.102	0.735	1.136	0.814	0.106	0.630	1.051
Log unscaled concurrent ACG score	1.810***	0.086	1.649	1.988	1.883***	0.117	1.666	2.128
Constant	1.910***	0.326	1.370	2.670	1.993***	0.421	1.318	3.014

Inflation model (0 PCP visits)
Dual eligibility: Medicaid 1–12 months	0.906**	0.029	0.850	0.964	0.926	0.042	0.847	1.011
Log unscaled concurrent ACG score	1.052	0.178	0.755	1.467	0.968	0.242	0.593	1.579
Constant (coefficient)	−0.710*	0.341	−1.370	−0.038	−1.066*	0.529	−2.102	−0.300
Log alpha (coefficient)	−1.150***	0.200	−0.162	−0.036	−1.103***	0.238	−1.57	−0.637
Alpha (coefficient)	0.318	0.063	0.215	0.470	0.332	0.079	0.208	0.529

Data source: Centers for Medicare and Medicaid, Medicare Limited Data Set 2010 file for 5% carrier file (professional service file).

Reference group for indicator variables: ASD-only, 18–21 years, white, male, no dual Medicaid coverage, no primary psychiatric claim in past year.

^* p < 0.05; ^**p < 0.01; ^***p < 0.001.

^a Number of observations = 527, number of zero observations = 168, Wald χ² = 142.2, p < 0.001.

^b Number of observations = 364, number of zero observations = 107, Wald χ² = 96.0, p < 0.001.

CI, confidence interval; IRR, incidence rate ratio; LR, likelihood ratio; SE, standard error.