Primary care databases provide rich consultation and prescribing information,¹ but lack individual background detail. Linking to information from questionnaires can overcome this, but requires individual informed consent.^2,3 The generalisability of linked data will be constrained by low consent rates and any differences between those giving and those withholding consent, in addition to any non-response bias. Most studies examining the consent issue, have either focused on patients with specific illnesses,³ or have had limited comparison measures.⁴ A recent large study found that men, younger people, and subjects with the symptom under investigation, were more likely to give consent, but lacked information on service use, or other measures of physical and psychological health.⁵ We conducted a wide ranging health and social survey and compared older people who gave and withheld consent to link questionnaire data to their primary care records.

PARTICIPANTS, METHODS, AND RESULTS

A detailed account of the survey methods has already been published.⁶ A postal survey was conducted, with an offer of assistance in completion, with patients aged ⩾65 years from two London practices. The questionnaire is available on request from the authors and is also on the journal web site (http://www.jech.com/supplemental) and included details on: demographic factors; physical health; psychological measures …