Table 1. Difference in means.

The data source is the universe of birth certificates in Pennsylvania (2004–2013) matched to the Pennsylvania DEP Internal Operator Well Inventory. Maternal and infant demographic indicators and health outcomes are recorded at the time of birth. “Near, 0–1 km” indicates that the mother lives within 0 to 1 km of at least one well site. “Far, 3–15 km” indicates that the mother lives 3 to 15 km from the nearest well site. Columns (5) to (7) report P values from t tests of equality of means across the different samples indicated. Column (7) tests whether (2) − (1) = (4) − (3). This quantity is referred to as the difference-in-differences, or D-in-D.

	Near, 0–1 km		Far, 3–15 km		P values
	Before	After	Before	After	(5)	(6)	(7)
	(1)	(2)	(3)	(4)	(1) − (2)	(3) − (4)	D-in-D
Mother characteristics
Married	0.68	0.61	0.62	0.63	0.00	0.00	0.00
Black	0.01	0.02	0.12	0.06	0.01	0.00	0.00
Hispanic	0.01	0.01	0.02	0.02	0.66	0.00	0.57
<High school	0.11	0.12	0.11	0.10	0.27	0.00	0.01
High school	0.28	0.31	0.26	0.25	0.02	0.00	0.00
Some college	0.32	0.32	0.28	0.29	0.89	0.00	0.43
College	0.22	0.17	0.22	0.23	0.00	0.00	0.00
Advanced	0.08	0.08	0.13	0.13	0.74	0.69	0.74
<20 years old	0.06	0.06	0.06	0.05	0.89	0.00	0.11
20–24 years old	0.21	0.26	0.22	0.21	0.00	0.00	0.00
25–29 years old	0.30	0.31	0.29	0.29	0.42	0.00	0.86
30–34 years old	0.26	0.23	0.27	0.28	0.01	0.00	0.00
>35 years old	0.17	0.14	0.17	0.16	0.01	0.00	0.07
Infant characteristics
Male	0.50	0.52	0.51	0.52	0.24	0.27	0.32
First born	0.42	0.41	0.43	0.43	0.66	0.15	0.84
Second born	0.33	0.34	0.33	0.33	0.46	0.05	0.68
Third born	0.16	0.16	0.15	0.15	0.79	0.64	0.73
Fourth and up	0.09	0.09	0.09	0.09	0.90	0.51	0.98
Health outcomes
Low birth weight	0.05	0.07	0.06	0.06	0.04	0.00	0.01
Birth weight	3354.35	3312.81	3316.94	3331.08	0.01	0.00	0.00
Health index	0.05	0.01	0.01	0.02	0.01	0.00	0.00
n	4871	1798	133,107	78,366