Reprinted from Australian Family Physician Vol. 32, No. 1/2, January/February 2003  95 T he first divisions of general practice were established in 1992 to provide clinical, management and professional support for general practitioners. 1 They have become increasingly influential local organisations. There are now 37 divisions of general practice in New South Wales alone, 20 urban and 17 rural. 2 Each year, the National Information Service (NIS) conducts an informative Annual Survey of divisions. 3 In 2000, the NIS estimated that 82% (n=18764) of all GPs in Australia were members of divisions. 3 Another national study had shown that 78% of GPs were members of a division. 4 Further analyses of unpublished data from this previous research 4 found that the proportion of GPs who were members of divisions by state ranged from 66% in Tasmania to 95% in South Australia. The NSW proportion was 74%. The only independent predictor of GP division membership was male sex. 4 A growing number of projects and other initiatives have been conducted using divisions as a sampling frame. 5–9 Using this method will miss some GPs because membership is not mandatory. Furthermore, membership of multiple GP divisions is also possible. We explored this further. Method Ethics approval was obtained from the CSAHS Ethics Review Committee. To obtain a sampling frame for our postal survey, we purchased names and contact details of all GPs in NSW from IMS Health (n=6951). 10 Using a random number software program we selected 550 names. Of these, 60 were ineligible: 11 were on extended sick leave, annual leave or maternity leave, six had retired, 23 were no longer in general practice and 20 had left with no forwarding address and were unknown to directory enquiries. In November 2000, the 490 remaining GPs each were mailed a self administered questionnaire about stroke diagnosis, management and prevention. We exam- ined division membership by asking: ‘How many GP divisions do you belong to?’ The following response options were given: ‘none’, ‘one’, ‘two’, ‘three’, ‘unsure’. Standardised follow up strategies for nonresponders were employed. Results We received 296 completed question- naires (response fraction 60%). General practitioners’ responses to our question regarding divisional membership are Contamination of interventional research is possible through GP membership of more than one division Sandy Middleton, Jeanette Ward Sandy Middleton, BAppSc, MN, FCN, (NSW), is a NH&MRC doctoral student, Division of Population Health, Central Sydney Area Health Service and School of Public Health, University of Sydney, New South Wales. Jeanette Ward, MBBS, MHPEd, PhD, FAFPHM, is Director, Division of Population Health, South Western Sydney Area Health Service, New South Wales. INTRODUCTION General practice divisions were promoted in the 1990s to provide support for general practitioners. Membership patterns are not well understood and may have implications for research and health services development. METHODS Within a postal questionnaire conducted in 1999, we determined self reported membership of divisions. RESULTS We obtained a 60% response rate (n=296) from a random sample drawn from all New South Wales GPs. The majority of GPs (n=204, 69%) belonged to one division. Thirty respondents (10%) did not belong to any division. Fifty-nine GPs (20%) belonged to two or more divisions, women GPs (n=27, 31%) significantly more than men (n=32, 16%) (P=0.002), and GPs with city or metropolitan area practices (n=52, 24%) were significantly more likely than rural or remote GPs (n=7, 9%) (P=0.005) to belong to two or more divisions. DISCUSSION If divisions are used as the unit of randomisation for interventional research, there is risk of contamination in study design. Articles reporting such trials should acknowledge this.