Google has no single authority metric but rather uses a bucket of signals to determine authority on a page-by-page basis. “We have no one signal that we’ll say, ‘This is authority.’ We have a whole bunch of things that we hope together help increase the amount of authority in our results,” said Paul Haahr, one of Google’s senior engineers who is involved with search quality. “Our goal in all of this is that we are increasing the quality of the pages that we show to users. Some of our signals are correlated with these notions of quality,” Haahr said. “We wouldn’t want to look at Twitter or YouTube as, ‘How authoritative is this site?’ but how authoritative is the user [i.e., individual user pages] on this site,” Haahr said. “Consider two articles on the same topic, one on the Wall Street Journal and another on some fly-by-night domain. Given absolutely no other information, given the information we have now, the Wall Street Journal article looks better. That would be us propagating information from the domain to the page level,” Haahr said.