Interpretation of England and Wales cancer mortality data: the effect of enquiries to certifiers for further information.

For some death certificates in England and Wales the cause information coded and published in national data is not that initially submitted by the certifier, but instead derives from a subsequent enquiry to the certifier for further information. These enquiries can lead to substantial artefacts in secular mortality data, and also to substantial non-comparability between mortality data for special study groups, such as subjects in cohort studies, and published mortality data. A description of current enquiry policy relevant to cancers, and changes in this policy over recent years is given to aid interpretation of mortality data. The effects on secular data of changes in enquiry policy are illustrated. At 4-digit level of the ICD, changes in enquiry policy can alter published mortality rates by several hundred per cent. At 3-digit level the greatest effects of enquiries at present are to increase the number of deaths coded to cancer of the eye by 35% and cancer of the body of the uterus by 31%; cancers of the thymus, heart and mediastinum are increased by 18%, and pleural cancer by 17%, while decreases of more than 10% are caused for several 'other' and 'unspecified' rubrics, and a decrease of 6% for deaths coded to melanoma.