Results
Summary Statistics. Table 1 reports the summary statistics for
several of the key determinants of mortality rates and provides
evidence on the validity of the RD design. Columns (1) and (2)
report the means in cities south and north of the Huai River line.
Column (3) reports the mean difference between the North and
the South. Column (4) also reports the difference, but this time it
is adjusted for a cubic polynomial in degrees north of the Huai
River so that it is a test for a discontinuous change at the Huai
River line. A direct test of the RD design’s identifying assumption
that unobservables change smoothly at the boundary is, of course,
impossible, but it would nevertheless be reassuring if observable
determinants change smoothly at the boundary. (This is analogous
to the test in randomized control trials that observable
determinants of the outcome are independent of treatment status.)
Column (5) reports the P values associated with the tests that
the differences in columns (3) and (4) are equal to zero.
Two key points emerge from this table. First, there are large
differences in TSP exposure among Southern and Northern
Chinese residents, but not for other forms of air pollution (e.g.,
sulfur dioxide and nitrous oxides) owing to the greater distances
that they travel. Second, there are substantial differences in the
determinants of mortality rates (e.g., temperature and predicted
life expectancy) between the South and North, but adjustment
for the cubic polynomial in latitude greatly reduces these differences
and causes them to become statistically insignificant.
This finding supports the validity of the RD design and the 2SLS
approach to inferring the causal relationship between TSPs and
life expectancy.