Google Search Engine Optimisation and their 80/20 Rule
http://stevenforsyth.com/articles/google-optimization.htm

Callan, David
stevenforsyth.com
2005

Abstract:

Google's increasing use of anti-spam features has meant that optimising websites for Google has become much harder and it's now not just a case of opening your websites source files in notepad, adding some keywords into your various HTML tags, uploading your files and waiting for the results. In fact in my opinion and I'm sure others will agree with me, this type of optimisation, commonly referred to as onpage optimisation will only ever be 20% effective at achieving rankings for any keywords which are even mildly competitive. Those of us who aced maths in school will know this leaves us with 80% unaccounted for.