EDITOR—Comber draws a distinction between hospital based registration systems and a population based registry and raises the possibility that we missed cases of colorectal cancer.1 2

The leaders of both the Northern Ireland cancer register and the colorectal cancer register have shared their data for several years. Those at the colorectal cancer register are aware that their ascertainment focuses on patients having surgical intervention. As our paper indicated,2 we focused our analysis on patients who had surgical intervention …