Chapter Review Mini-Case Google's PageRank Google's PageRank is an algorithm that attempts to inform you where people are coming from when they land on your Website and which sources are the most frequent. Note that, as its name suggests, a page rank is an index estimated page by page; it's not an overall Website assessment. Nevertheless, say you're trying to determine the rank of your home page. You figure that's a good start, and customers can navigate more precisely once they're in your domain. The ranking model begins by checking all the incoming links to the home page over some given duration (say, the last 24 hours or the last week, depending on the site traffic and how current the information must be). Customers can land on the home page starting from many links, and the links generating traffic to you differ in their importance. In particular, the influence of the incoming pages varies, as weighted by two factors: The page rank of the source link (higher is better) How many outbound links that source page contains (A lower number is better, in that the link to your home page is therefore more selective.) Thus, if page A contains a link to your home page, and it has a high page rank of its own and relatively few outreaching links, it carries more weight than page B with its lower page rank and more outreach links. This algorithm is obviously iterative because we need to estimate the ranks of pages A and B before we can bring them into the estimation of the rank for your home page. In theory, the iterations could continue ad infinitum. In the actual algorithm, there are starting values, and about 100 iterations bring most estimates to the convergent approximation. Finally, Google then exercises the universal modeling prerogative of including a term for wiggle room or a fudge factor.