Submitted on: 26 June 2003.
Revised on: 14 September 2003.
Accepted on: 24 October 2003.
___________________International Journal of Biostatistics
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Submitted on: 19 June 2008.
Revised on: 11 September 2008.
Accepted on: 21 October 2008.
___________________International Journal of Biostatistics
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