Sounds to me like they score it. A long way from an exact science.
http://www.dtnprogressivefarmer.com/dtnag/common/link.do?symbolicName=/free/news/template1&paneContentId=5&paneParentId=70104&product=/ag/news/topstories&vendorReference=b88006fa-b53c-4980-88e5-e3a4e3a4d33e