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Submitted on: 16 October 2003.
Revised on: 22 January 2004.
Accepted on: 12 February 2004.
___________________BMC Medical Research Methodology
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Submitted on: 27 December 2002.
Revised on: 30 January 2003.
Accepted on: 27 March 2003.
___________________BMC Medical Research Methodology
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