Contamination of interventional research is possible through GP membership of more than one division.
BACKGROUND
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.


CONCLUSIONS
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.
Middleton
Sandy
S
Division of Population Health, Central Sydney Area Health Service, School of Public Health, University of Sydney, New South Wales.
Ward
Jeanette
J
D000328:Adult; D005194:Family Practice / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D008297:Male; D009517:New South Wales; D012107:Research Design; D012955:Societies, Medical / Q000706:statistics & numerical data*