The authors were aware of the relative inaccuracy of postcodes (used to define the location of cases and births), as these give only an approximation of place of residence, accurate to 10–100 m in urban areas but > 1 km in some rural areas. Furthermore, it is well known that postcodes are afflicted with several other problems: they may change over time, be terminated, or even recycled. However, such problems affect only a small minority (approximately 1%) of U.K. post-codes. Thus, given the size of the study (national rather than local), this is not a major problem. For further details the reader is referred to the original article (Elliott et al. 2001).