- Identify the top 200-400 search terms on your website. These top 200-400 terms will usually represent a very significant percentage of search volume.
- Identify the correct search result for each of these search terms.
- Test each of these top search terms in your search engine.
- Measure how often the correct result appears in the first three results. (Research shows that 60% of people will click on one of the first three results.)
- Compile a success rate. You will then be able to say something like: Our search has a 45 percent success rate. Then you can plan for how you will increase that success rate.
http://www.gerrymcgovern.com/nt/2007/nt-2007-05-14-search.htm