The Google search engine is based on measuring the value of a web site by the number of other sites linking to it and their values. Google’s founders came up with an efficient algorithm for evaluating these values (it is complicated because each site’s value depends on all the other site’s values, so none of them can be calculated first).
Trouble is, Google isn’t an innocent bystander in all this. So many people use Google that they MUST be influencing how successful sites are, and therefore indirectly influencing how many other sites link to them. Because they are influencing the human choices site managers and users make while putting content into their sites, the exact role Google played in helping a site get linked to isn’t explicitly quantifiable. Therefore they have become a somewhat inscrutable positive feedback node in this system they purport to measure.
Is there some mechanism by which they deal with this?