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Bernard, Andrew, Redding, Stephen and Schott, Peter (2007) Comparative advantage and heterogeneous firms. Review of economic studies, 74 (1). pp. 31-66. ISSN 0034-6527
Burchardt, Tania and Le Grand, Julian (2002) Constraint and opportunity: identifying voluntary non-employment. 55. Centre for Analysis of Social Exclusion, London School of Economics and Political Science, London, UK.