Spam mass is defined as "the measure of the impact of link spamming on a page's ranking." The concept was developed by Zoltán Gyöngyi and Hector Garcia-Molina of Stanford University in association with Pavel Berkhin and Jan Pedersen of Yahoo!.
TrustRank is a link analysis technique described in a paper by Stanford University and Yahoo! researchers for semi-automatically separating useful webpages from spam.