Submitted on: 14 October 2000.
Revised on: 17 January 2001.
Accepted on: 10 February 2001.
___________________BMJ-British Medical Journal---BMJ PUBLISHING GROUP
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Submitted on: 20 April 2018.
Revised on: 02 June 2018.
Accepted on: 24 July 2018.
___________________BMJ-British Medical Journal---BMJ PUBLISHING GROUP
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Submitted on: 08 February 2013.
Revised on: 11 March 2013.
Accepted on: 03 April 2013.
___________________BMJ-British Medical Journal---BMJ PUBLISHING GROUP