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By Charlie Karlsson, Roger Stough, Börje Johansson. Roger R. Stough is at George Mason University, USA. Currently, there is a considerable amount of confusion and a variety of conflicting theories which are being used interchangeably and ambiguously.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.