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This is an Author's Accepted Manuscript of an article published by Routledge (a subsidiary of Taylor & Francis) in Applied Economic Letters. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Applied Economic Letters, 19(10): 935-938, 2012 and can be found at http://dx.doi.org/10.1080/13504851.2011.608633 (Please note: access via this link may only be available with a subscription)