They link to their paper on the Google Correlate site: <a href="https:&#x2F;&#x2F;www.google.com&#x2F;trends&#x2F;correlate&#x2F;nnsearch.pdf" rel="nofollow">https:&#x2F;&#x2F;www.google.com&#x2F;trends&#x2F;correlate&#x2F;nnsearch.pdf</a>