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Submitted on: 19 April 2004.
Revised on: 24 May 2004.
Accepted on: 15 July 2004.
___________________Econometrics Journal---OXFORD UNIV PRESS
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Submitted on: 18 September 2003.
Revised on: 12 November 2003.
Accepted on: 11 December 2003.
___________________Econometrics Journal---OXFORD UNIV PRESS
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