RT Journal
A1 Waldman M, Nicholson S
T1 Autism prevalence and precipitation: The potential for cross-level bias—reply
JF Archives of Pediatrics & Adolescent Medicine
JO Archives of Pediatrics & Adolescent Medicine
YR 2009
FD May 4
VO 163
IS 5
SP 492
OP 493
DO 10.1001/archpediatrics.2009.84
UL http://dx.doi.org/10.1001/archpediatrics.2009.84
AB 
Their first criticism is that we inadequately control for differences in urbanicity across counties. We used 3 different statistical approaches to test whether precipitation was positively associated with autism prevalence rates, including a county fixed-effects (within county) specification. The fixed-effects regressions test showed that county-level birth cohorts exposed to high levels of precipitation, relative to the county mean, when younger than 3 years had higher autism prevalence rates relative to the county mean. That we find statistically significant results in these regressions indicates that our findings are not due to differences in urbanicity across counties, because urbanicity did not change much within counties during this period, and any changes are unlikely to be correlated with precipitation.