Drs Sinclair and Fairley raise two issues about our paper1 in their article, which they allege we did not resolve in our response2 to prior critiques, including one by EPA (unpublished data). They raise the same critiques about the accompanying editorial.3 Firstly, they raise the hackneyed bugaboos about ecological studies. These are issues in cross sectional comparisons of different populations with different exposures. But in our studies, the population of Philadelphia is compared not with another population, but with itself at different points of time. Hence, the population is its own control, as described in Schwartz and Levin.2

Their second argument is that there is too much measurement error in our exposure index, and therefore our reported association cannot be real. But there is an observed association between hospital admissions for gastrointestinal …