As with anything that requires optimisation, a scientific process is essential. SEO is no different, and you will struggle to improve your performance in search engines if you don’t take the time to analyze what has and hasn’t worked previously for your website, as well as what is working for your competition.
What is considered best practice is a constantly evolving theory. SEO that was considered good practice a few years ago, such as a strong keyword density and lots of meta keywords, can actually hurt your website if employed today.
Search engines keep the algorithms they use to decide where you rank on their systems closely guarded. Of course this does not stop people testing the platforms to find out what effects their ranking and to what degree. As of this moment there are over 200 factors that people have identified as having an impact on their rankings.