title: Three authors reply.
authors.affiliation: Health Protection Services: Local and Regional Services, Health Protection Agency, Chilton, Oxfordshire OX11 0RQ, UK.
authors.forename: Noel D
authors.initials: ND
authors.lastname: McCarthy
authors.forename: Iain A
authors.initials: IA
authors.lastname: Gillespie
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: O'Brien
mesh_terms: D016022:Case-Control Studies; D015340:Epidemiologic Research Design; D006801:Humans; D018570:Risk Assessment / Q000379:methods*