In the remainder of this section, we check if there are differences in observable characteristics across the VLBW cutoff by estimating our baseline model with the covariates as dependent variables. If the RD design is valid, then there should be no discontinuities at the VLBW cutoff.16 Table 1 provides the results. Panels A, C, and D use the FC/parent sample and check whether focal child and parental characteristics are balanced, while panel B uses the sibling sample to check for discontinuities in the covariates of siblings. Column 1 provides the conventional point estimate from the local-linear regression, and columns 2 and 3 present the bias-corrected estimate and the robust standard error (Calonico et al., 2019). We report sharpened q-values, that is, p-values based on the numbers in columns 2 and 3 further corrected for multiple inference and in column 4 (see section III and Anderson, 2008, for details). Finally, column 5 reports the mean of the covariate in the sample of (family members of) focal children with birth weight of at least 1,500 g. The results show that observations just below the VLBW cutoff are generally similar to those just above the VLBW cutoff. There are few characteristics with marginally significant discontinuities at the threshold, and none of these survive the adjustment for multiple inference: the lowest q-value is 0.5.

Table 1.

Distribution of Covariates across the VLBW Cutoff

EstimateBias-Corrected EstimateRobust Standard ErrorSharpened q-ValueMean of Dependent Variable
(1)(2)(3)(4)(5)
A. Focal child characteristics (N = 2,156)
Boy −0.028 [−0.079] (0.077) {0.919} 0.456
Birth order 0.229 [0.166] (0.173) {0.919} 1.911
Multiple birth 0.065 [0.092] (0.070) {0.851} 0.208
Gestational age −0.353** [−0.508] (0.258) {0.576} 34.097
Family size 0.054 [0.009] (0.160) {1.000} 2.937
Has younger siblings −0.066 [−0.020] (0.078) {1.000} 0.611
Number of younger siblings −0.189 [−0.205] (0.133) {0.713} 0.899
B. Sibling characteristics (N = 3,311)
Boy −0.003 [−0.033] (0.068) {1.000} 0.520
Birth order −0.115 [−0.154] (0.147) {0.919} 2.121
Multiple birth 0.026 [0.011] (0.017) {1.000} 0.023
Gestational age −0.319 [−0.464] (0.449) {0.919} 38.209
Birth weight −128.494* [−188.938] (105.751) {0.618} 2,898.7
VLBW 0.012 [0.019] (0.033) {1.000} 0.046
Age difference–older sibling −0.119 [−0.397] (0.782) {1.000} 6.586
Age difference–younger sibling −0.400 [−0.691] (0.449) {0.713} 4.515
C. Mother's characteristics at the birth of the focal child (N = 2,156)
Age 1.118 [1.040] (0.800) {0.851} 27.735
Education (years) −0.246 [0.218] (0.389) {1.000} 11.239
Immigrant −0.021** [−0.052] (0.027) {0.576} 0.068
Married 0.047 [0.003] (0.080) {1.000} 0.535
D. Father's characteristics at the birth of the focal child (N = 2,116)
Age 2.044** [2.132] (0.873) {0.507} 30.547
Education (years) 0.172 [0.465] (0.400) {0.919} 11.608
Immigrant 0.013 [−0.006] (0.039) {1.000} 0.074
Not reported −0.008 [−0.004] (0.023) {1.000} 0.021
EstimateBias-Corrected EstimateRobust Standard ErrorSharpened q-ValueMean of Dependent Variable
(1)(2)(3)(4)(5)
A. Focal child characteristics (N = 2,156)
Boy −0.028 [−0.079] (0.077) {0.919} 0.456
Birth order 0.229 [0.166] (0.173) {0.919} 1.911
Multiple birth 0.065 [0.092] (0.070) {0.851} 0.208
Gestational age −0.353** [−0.508] (0.258) {0.576} 34.097
Family size 0.054 [0.009] (0.160) {1.000} 2.937
Has younger siblings −0.066 [−0.020] (0.078) {1.000} 0.611
Number of younger siblings −0.189 [−0.205] (0.133) {0.713} 0.899
B. Sibling characteristics (N = 3,311)
Boy −0.003 [−0.033] (0.068) {1.000} 0.520
Birth order −0.115 [−0.154] (0.147) {0.919} 2.121
Multiple birth 0.026 [0.011] (0.017) {1.000} 0.023
Gestational age −0.319 [−0.464] (0.449) {0.919} 38.209
Birth weight −128.494* [−188.938] (105.751) {0.618} 2,898.7
VLBW 0.012 [0.019] (0.033) {1.000} 0.046
Age difference–older sibling −0.119 [−0.397] (0.782) {1.000} 6.586
Age difference–younger sibling −0.400 [−0.691] (0.449) {0.713} 4.515
C. Mother's characteristics at the birth of the focal child (N = 2,156)
Age 1.118 [1.040] (0.800) {0.851} 27.735
Education (years) −0.246 [0.218] (0.389) {1.000} 11.239
Immigrant −0.021** [−0.052] (0.027) {0.576} 0.068
Married 0.047 [0.003] (0.080) {1.000} 0.535
D. Father's characteristics at the birth of the focal child (N = 2,116)
Age 2.044** [2.132] (0.873) {0.507} 30.547
Education (years) 0.172 [0.465] (0.400) {0.919} 11.608
Immigrant 0.013 [−0.006] (0.039) {1.000} 0.074
Not reported −0.008 [−0.004] (0.023) {1.000} 0.021

Sample of (family members of) focal children with birth weight within a 200 g bandwidth around the 1,500 g cutoff and gestational age of at least 32 weeks. Column 1 reports the estimated coefficient of the VLBW variable from a separate local-linear regression with a triangular kernel of the characteristic listed in the row for the family member indicated in the panel heading. Column 2 reports the corresponding bias-corrected estimate, column 3 the robust standard error (Calonico et al., 2014, 2019), column 4 the sharpened q-value for the set of tests included in the table (Anderson, 2008), and column 5 the mean of the variable in the row calculated among (family members of) focal children with birth weight above 1,500 g. Asterisks indicate statistical significance (significant at **5% and *10%), based on robust confidence intervals centered on the bias-corrected estimates (for details, see Calonico et al., 2014, 2019).

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