Every two years, Moz surveys over 100 top industry professionals to compile our biennial Search Engine Ranking Factors. For 2013, we've supplemented the survey with real-world correlation data from a scientific examination of over 17,000 keyword search results by Dr. Matt Peters and his data science team.

We've released some of the 2013 data previously, but not the full set until now. So with great pleasure, I present the complete results of this year's survey and correlation data:

2013 Search Engine Ranking Factors

Why do we call it ranking factors?

Google claims to use over 200 signals in its search algorithm. While we don't know what exactly these signals are, it's helpful to examine high-ranking pages so that we can begin to understand the characteristic of pages search engines like to reward. This can give us a "hint" as to the ranking factors the search engines actually use.

"Correlation is not causation but it sure is a hint."
- Edward Tufte

For example, if the correlation data shows us that high-ranking pages are associated with a high number of external backlinks, we might guess that backlinks are still an important part of Google's algorithm.

On the other hand, the correlation data doesn't always point us toward a direct ranking factor (causation), but instead only points us towards the characteristics of high-ranking pages. For example, this year's data shows an amazing correlation between the number of Google+1's and higher rankings, yet Google representatives state +1's aren't used in the algorithm.

This doesn't mean sharing on Google+ isn't important, but does tell us we should pay closer attention to try to understand why pages with a lot of +1's are also pages that tend to rank higher in search results.

This post and chart by Rand Fishkin helps illustrate the difference between correlation and causation.

Comparing correlation metrics

This year's Ranking Factors contains an interactive chart that allows you to compare different categories of correlations against each other, such as in this snippet comparing page-level link metrics (yellow) to domain-level link metrics (blue).

By comparing the data, we find that high-ranking URLs are more highly correlated with page-level link metrics than general domain-level link metrics.

What's really incredible about this view is that Page Authority is the most highly correlated metric in this year's study at 0.39, making it one of the most highly correlated SEO metrics we've ever observed.

On-page keyword usage vs. page-level anchor text

While the correlation with on-page keyword usage has declined over the years, the correlations with page-level anchor text remain as strong as ever. For instance, the number of root domains linking to the page with partial match anchor text has a 0.29 correlation.

Social correlations vs. link metric correlations

Social metrics have also gained a lot of attention in past years. Do they still correlate well with higher rankings? This chart compares social metric correlations with traditional link correlations.

Here we see social metric correlations almost equal with link metric correlations. This almost perfectly illustrates the need to not jump to conclusions with correlation data, for while we suspect Google may use social data in its search algorithm, it's also true that pages that get a large number of social shares also tend to earn a high number of links.

Just because a metric is highly correlated, doesn't mean Google uses that metric directly.

The future of search: 128 industry experts lead the way

Raw data only gets you so far. Often, the observations and experience from those on the ground goes much farther when working toward search marketing success.

This year, we invited over 100 industry experts to weigh in on how they see search engines working, what tactics are successful, and what the future might hold.

We asked each of our contributors about the future of search. The chart below shows where our experts think things are headed.

Real world, in-depth insight

We also asked each expert to weigh in on each survey question. Here are a few responses from the Future of Search question.

Dev Basu
"Social signals from Google+ will become more and more relevant over the next 12 months. Eventually Google+ will take equal, if not more, prominence than Facebook due to its position at the cross-section of search and social."

Laura Lippay
"Some of the research papers from universities and/or search engines have shown them testing things like how long the window is in focus, scrolling on a page, printing from a page, and-what I think is the most interesting one (used for testing a page's credibility)-automating the process of identifying topical experts and giving weight to the pages they visit."

Todd Malicoat
"Don't believe the hype of social and G+. They will increase in importance, but certainly not replace more traditional offsite factors. Inbound links and offsite equity will continue to be highly influential to search relevance when validated by other factors. Expect these offsite equity trust and authority factors to maintain a large portion of the importance that they've always held as the foundation of the Google algorithm."

Read more expert comments and see the full data results in the 2013 Search Engine Ranking Factors.

Bonus: Interested in the historical data to see how factors have changed?

• 2011 Search Engine Ranking Factors
• 2009 Search Engine Ranking Factors