Spam Score represents the percentage of sites with similar features found to be penalized or banned by Google. Spam Score is based on Moz's machine learning model which identified 27 common features among the millions of banned or penalized sites in the data they fed it.
The old version of the spam score was based on 17 points of evaluation. With Moz's new Link Explorer now there are 27 "flags". Each "flag" is a potential "cautioning sign" that a site may be spammy. The higher the number of flags on a link, the higher the chance that it's spammy.
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