Table 4.

Second-stage Estimates of Two-stage Least Square IV Estimation of WIC Participation on Ever Breastfeeding in a WIC-eligible Population (Income ≤ 230% of the FPL) ^†

OLS						IV: SNAP+SNAP2+SNAP policy index
	Coeff.	S. E	P-value	Elasticity estimate (at the mean of breastfeeding probability)††	Coeff.	S. E	P-value	Elasticity estimate (at the mean of breastfeeding probability)††	O-stat	P-value	Identification test‡
ALL	−0.064	0.007	< 0.001	−0.011	0.085	0.298	0.775	0.014	3.951	0.139	UWO
Boy	−0.072	0.009	< 0.001	−0.012	0.444	0.384	0.247	0.075	4.332	0.115	O
Girl	−0.057	0.009	< 0.001	−0.010	−0.147	0.367	0.689	−0.025	0.930	0.628	UWO
Non-H White	−0.084	0.008	< 0.001	−0.014	−0.425	0.532	0.424	−0.070	5.863	0.053	O
Non-H Black	−0.071	0.018	< 0.001	−0.017	0.506	0.679	0.456	0.118	0.590	0.745	UO
Hispanic	−0.026	0.012	0.040	−0.004	0.614	0.290	0.034	0.093	1.956	0.376	O
Other	−0.042	0.016	0.011	−0.007	−0.146	0.500	0.771	−0.024	2.330	0.312	UO

^†Controlling state dummy, sex, race/ethnicity, education, child’s age, maternal age, maternal marital status, number of people in the household, interview language, birth year, TANF/Temporary Assistance for Needy Families enrollment rate (# of enrolled participants/# of state population), CHIP/Children’s Health Insurance Program enrollment rate (# of enrolled children/# of children 18 or under in the state), and state unemployment rate. IV Variables including SNAP enrollment rates (SNAP, # of SNAP participants in that year/# of state population in that year), square of SNAP enrollment rates (SNAP2), and SNAP policy index.

^††Elasticity estimates were obtained from the marginal effect estimates and the breastfeeding and WIC participation means using the elasticity formula.

^‡U means under-identification test P < 0.05. Stock–Yogo tabulations based on the Cragg–Donald statistic and used for the weak identification test; W means the F statistic of the weak identification test > 10% maximal IV relative bias or > 10% maximal IV size; O means over-identification test > 0.05.