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Donald Trump has 55 million followers on Twitter. But his tweets reach less than half of that, according to an analysis by SparkToro.

More than 60% of the US president’s Twitter following, or about 33 million accounts, have been deemed fake by the marketing software firm. It ran all of Trump’s followers through a machine-learning based tool that picks up signals that an account might be spam, propaganda, a bot, or inactive. If the account racks up seven to 10 signals or more, it was counted as fake.

What counts as a signal? Here are some that SparkToro’s tool registered for Trump’s followers:

72% have been inactive for 120+ days

3% have been inactive for between 90-120 days

3% created their account in the last 90 days

36% use Twitter’s default profile image

39% use display names that include spam words and patterns

92% either don’t use a URL in their profile or employ a URL with spam patterns

60% don’t use a recognized location

27% have set their language to something other than English

54% have gone more than a year without sending more than a handful of tweets

3% send an abnormally large number of tweets per day

96% have been placed on very few (or zero) lists

79% have an unusually small number of followers

76% follow an unusual number of accounts

74% employ spam-correlated keywords in their profile description

Here’s how SparkToro’s founder, former Moz chief executive Rand Fishkin, explains the rationale behind the analysis:

I have this sinking feeling that someone will undoubtedly point to one of these signals and say “but that doesn’t mean the account is spam/bot/propaganda/inactive!” so let me state this again: no one, two, three, or even six signals means we’ll treat an account as low quality. But, I think we’d all agree (and the machine learning model built off spam+bot accounts would concur) that it’s very rare that an account could have a combination of 7-10+ of the above signals and still be a real, active, human being regularly logging into their Twitter account. Not impossible, just exceptionally unlikely.

SparkToro also looked at the following of other prominent US politicians. Though Trump has the highest share of fake followers by far in the sample, it’s a problem that spans geographies and party lines. Here are the results:

The data offer a reality check for politicians, but they’re not necessarily a reflection of their reach online, SparkToro points out, particularly in the case of the president. While Trump may only be reaching 25 million human-run Twitter accounts, he can count on the media and others to spread his tweets to a much broader group.