The New York Times has recently published an article about Amit Singhal. Amit Singhal is in charge of Google's ranking algorithm. The interview reveals some interesting facts about Google's ranking algorithm.
Google knows that its algorithm is not perfect
"Tweaking and quality control involve a balancing act. 'You make a change, and it affects some queries positively and others negatively,” [...] 'You can’t only launch things that are 100 percent positive.'"
"[...] Any of Google’s 10,000 employees can use its 'Buganizer' system to report a search problem, and about 100 times a day they do."
Why Google changes its algorithm
The article lists a concrete example why Google could change its algorithm:
"Recently, a search for 'French Revolution' returned too many sites about the recent French presidential election campaign — in which candidates opined on various policy revolutions — rather than the ouster of King Louis XVI.
A search-engine tweak gave more weight to pages with phrases like 'French Revolution' rather than pages that simply had both words."
If you want to get high rankings on Google, it's important to know whether you should use your keywords as a phrase or as separate words on your web pages.
This can be different for different keywords. Fortunately, there is a way to find out how you should use your keywords on your web pages (see below).
PageRank is just one of many factors
While PageRank was very important when Google was new, Google now uses many more factors to determine the rankings of web pages:
"PageRank is but one signal. Some signals are on Web pages — like words, links, images and so on. Some are drawn from the history of how pages have changed over time. Some signals are data patterns uncovered in the trillions of searches that Google has handled over the years."
There are many factors that influence the ranking of a web page on Google. If you want to get high Google rankings for your website, then you have to work on all of these factors.