Probably best to start here with the original paper and then search for related material as the idea has been kicked around for a while: http://www.marketingvp.

Probably best to start here with the original paper and then search for related material as the idea has been kicked around for a while: http://www.marketingvp.com/download/mer-data.pdf http://www.marketingvp.com/download/mer-data.pdf