TABLE 2—

Utilization of Physician Services Before and After the July 1, 1966, Implementation of Medicare/Medicaid by Age and Income Groups: United States

		Unadjusted Results				Adjusted Results
	No.	Before	After	Difference	P	DiD Estimate (95% CI)	P
Doctor visits/100 persons
By age, y
< 65	485 292	406.5	407.7	1.1	.79	0 (Ref)
≥ 65	48 982	624.9	590.8	−34.1	.09	−26.8 (−67.6, 14.0)	.2
By income
Bottom tertile	174 636	358.0	380.2	22.3	.002	52.1 (28.6, 75.5)	< .001
Middle tertile	171 822	433.0	428.0	−5.0	.55	15.0 (−10.1, 40.1)	.24
Top tertile	160 157	501.5	478.0	−23.5	.01	0 (Ref)
By income, among the elderly
Bottom tertile	24 500	561.9	585.3	23.4	.34	105.1 (−10.0, 220.1)	.07
Middle tertile	11 545	671.4	598.4	−73.1	.07	11.7 (−125.2, 148.6)	.87
Top tertile	9119	717.4	639.4	−78.0	.15	0 (Ref)
By combined target status: age ≥ 65 y or bottom income tertile
Not target	311 315	449.8	441.5	−8.3	.18	0 (Ref)
Target	199 118	399.3	407.5	8.2	.28	26.9 (6.4, 47.4)	.011
Surgical procedures/100 persons
By age, y
< 65	738 208	7.05	6.65	−0.40	< .001	0 (Ref)
≥ 65	74 120	6.50	7.45	0.95	.002	0.90 (0.31, 1.49)	.003
By income
Bottom tertile	270 464	6.73	6.72	−0.01	.95	0.36 (−0.09, 0.81)	.12
Middle tertile	255 453	7.50	7.09	−0.41	.008	−0.13 (−0.59, 0.33)	.58
Top tertile	246 028	6.92	6.58	−0.34	.046	0 (Ref)
By income, among the elderly
Bottom tertile	36 833	5.70	6.68	0.98	.005	−0.41 (−1.81, 0.99)	.57
Middle tertile	17 368	7.27	7.55	0.28	.64	−1.07 (−2.79, 0.65)	.22
Top tertile	14 327	7.68	9.11	1.43	.032	0 (Ref)
By combined target status: age ≥ 65 y or bottom income tertile
Not target	469 786	7.20	6.74	−0.46	< .001	0 (Ref)
Target	307 751	6.79	6.91	0.11	.45	0.51 (0.16, 0.85)	.005

Note. CI = confidence interval; DiD = difference-in-difference. Adjusted models included variables for age, sex, family size, region, race, marital status, education, employment, income category, and a pre–post 1966 expansion dummy (Appendix, note 5). Adjusted DiD models “by age” also included an age-65-years-and-older dummy variable and an interaction term between this variable and the pre–post expansion dummy variable; these models excluded the continuous age variable. Adjusted DiD models “by income” also included the income tertile variable and an interaction term between this variable and the pre–post 1966 expansion dummy variable; these models excluded the categorical income covariate (see Appendix, notes 5 and 6, for details on models and treatment of covariates). A total of 345 individuals with weight = 0 were excluded from all analyses.

Source. Authors’ analysis of the National Health Interview Survey for various years. For office visits, the “before” year was 1963–1964 and “after” years were 1966–1967, 1967–1968, and 1969. For surgical procedures, “before” years were 1963–1964, 1964–1965, and 1965–1966, and “after” years were 1966–1967, 1967–1968, and 1969.