@article {Portela2010,
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	number = {3},
	year = {2010},
	pages = {618-639}
}
@article {Portela2001,
	title = {Measuring skill: A multi-dimensional index},
	journal = {Economics Letters},
	volume = {72},
	number = {1},
	year = {2001},
	pages = {27-32}
}
