BMJ  2005;331:1082-1083 (5 November), doi:10.1136/bmj.331.7524.1082-c

Letter

Are consenters representative of a target population?

EDITOR—I write with reference to the paper by Junghans et al on recruiting patients to medical research.¹ We recently published a method to determine if consenters to population surveys are representative of the target study population.²

Recently introduced data protection legislation has changed research practice in the United Kingdom, although uncertainties about the interpretation of the legislation remain.³ Methods such as searching general practice records to investigate whether non-responders are different to responders are no longer acceptable as they use information about identifiable individuals without their consent. An alternative approach to estimating the effect of non-response is to reformulate the problem as the extent to which responders are representative of the total target population. Since anonymised data may be obtained and used for the target population as a whole, this represents an indirect way of assessing the impact of non-response on representativeness. We compared consultation rates for some common conditions among responders to a headache survey⁴ who had consented to the use of their practice records for research with anonymised rates derived from the practice populations from which they were sampled.

We found that the adjusted consultation rates were similar but generally higher in the consenters group than in the population group. This shows that consenters have more morbidity than those who do not participate, not only with respect to the topic of the survey (headache) but for other conditions also. However, the differences were not large and imply that non-response bias is unlikely to be substantial with respect to the extent and pattern of ill health.

The alternative method that we described offers one potential approach to determine whether respondents to a study are representative of the population from which they were sampled with respect to a particular set of characteristics—namely, their morbidity as measured by consultation with a general practitioner—and as such can be used as one measure of the representativeness of the responding sample.

Leek Health Centre, Fountain Street, Leek ST13 6JB david.millson{at}tiscali.co.uk

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Competing interests: None declared.

References

1. Junghans C, Feder G, Hemingway H, Timmis A, Jones M. Recruiting patients to medical research: double blind randomised trial of "opt-in" versus "opt-out" strategies. BMJ 2005;331: 940. (22 October.)[Abstract/Free Full Text]
2. Boardman HF, Thomas E, Ogden H, Croft PR, Millson DS. A method to determine if consenters to population surveys are representative of the target study population. J Public Health Med 2005;27: 212-4.[Abstract/Free Full Text]
3. Strobl J, Cave E, Walley T. Data protection legislation: interpretation and barriers to research. BMJ 2000;321: 890-2.[Free Full Text]
4. Boardman HF, Thomas E, Croft PR, Millson DS. Epidemiology of headache in an English district. Cephalalgia 2003;23: 129-137.[CrossRef][ISI][Medline]

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