The report was based on data from a survey conducted in August 2019 among 1,584 SEO professionals. Respondents were asked their opinions on how much weight 26 factors receive in Google search's organic ranking systems (the classic, 10 blue links style results). Each expert rated each factor on a 0-10 scale (0=not used; 5=moderately weighted; 10=very heavily weighted).
Relevance of page content receives the highest mean rating (8.52) among the 26 factors; the quality of linking sites is second (7.87), and the use of query-relevant words and phrases is third (7.5).
The researchers also looked at which factors SEO experts agree on and disagree on most by examining the standard deviation among responses (i.e., how tightly clustered or spread out responses were around a number).