Because the search engines now routinely track web pages numbering in the billions, it is important that they develop systems to decide on the trustworthiness of information on the internet.

To do this, they feed information about specific web pages into an 'algorithm'. This is simply a set of rules that enables a computer program to make a decision about the quality and relevance of a web page.

The best known search engine algorithm is that of Google.

Some Factors Used in Google's Alogorithm

The precise combination of factors used by Google's algorithm is a closely guarded secret - and also ever evolving.

Most search engine optimisers who have experience agree on a large number of the factors that are used in the algorithm - although the weight accorded to each one is a hotly debated subject.

Below is a brief list of some of the factors that are thought to be taken into account by the Google algorithm.

Domain Factors

1. The age of the domain.
2. The number of links to that domain in total
3. The source of those links
4. The age of those links
5. The history of the domain (frequently updated over a long period)
6. Association of the domain with 'malware', suspicious activities etc.

'On page' factors

1. Readability of the page
2. Uniqueness of content
3. Age of content
4. Broken links
5. Loading time
6. The page title
7. Meta tags - particularly the meta description

Further Reading

• Information from Google
• Part of Google's Algorithm Revealed
  SEOmoz report on the partial leak of Google's algorithm
• Original Google Paper
  The original paper by Sergey Brin and Lawrence Page describing how the Google algorithm would work.