SEO Salaries

SEOMoz has an interesting post on how much SEO's make yearly.

Search Engine Optimization (SEO) Salaries seem to be growing more and more lately, because everyone wants to be number one in the search engines.

SEOMoz posted an article on SEO salaries and how much each position makes on average. Each position is sectioned into either In-House SEO's or SEO Agency Employees. Rand Fish also includes "factors that can create wide disparity."

Overall, I'm wondering how the research was conducted. Was it based on a forum poll, or was it just estimating by personal experiences and interactions?

I'm just curious. :-)