Submitted on: 04 February 2008.
Revised on: 31 May 2008.
Accepted on: 16 July 2008.
___________________ECONOMICS LETTERS---ELSEVIER SCIENCE SA
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Submitted on: 29 April 2024.
Revised on: 28 May 2024.
Accepted on: 23 July 2024.
___________________ECONOMICS LETTERS---ELSEVIER SCIENCE SA
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