Matt Cutts, Google software engineer has clearly stated that W3C validation does not affect search engine rankings. In other words, error free code as measured by the W3C code validator is not rewarded with a higher search ranking by Google. This in itself is not a surprise.

Most SEO firms take the position that clean code does matter in organic SEO. Clean code helps the search engine spiders read and understand your website easier.  Websites filled with hundreds of errors will perform worse than websites with relatively clean code.

I decided to test that assumption. I took 14 different keyword phrases from different markets and industries. I decided to compare top ranked websites (ranked 1-5) against lower ranked sites (ranked 101-105) and also against even lower ranked web pages (ranked 201-205). Google was used for the search results.

I ran every web page that appeared in the Google search results through the W3C validator and tabulated the results.

Results:

1. 210 web pages were checked for errors (14 phrases x 15 positions each).
2. 25,813 total errors were detected using the W3C validator.
3. 123 errors per page average was found. This ranged from 0 errors (valid code) to 1403 errors Position 105 for the phrase “amusement parks.”
4. 4.28% were valid code: 9 pages were found to be valid code (4.28%). This is close to the Opera study of 3.5 million pages that found 4.13% of all pages checked to be valid.
5. 5.7% more errors in 1-5: Positions 1-5 had 130 errors per page which is 5.7% more than the average for all pages checked. The best coded sites were actually found in positions 201-205. See the details below.

Conclusion: Matt Cutts was correct in his statement that Google does not use W3C validation as part of their ranking algorithm.