Twitter uses reCAPTCHA, phone number verification, or email address verification to check if a user is human.
As part of its efforts to prevent manipulation of the platform by bots and fake accounts, Twitter challenged 19.35 crore accounts for verification in the second half of 2018, 17 percent fewer than the first half of last year, according to a transparency report from the micro-blogging site.
Twitter uses the "anti-spam challenge" for confirming whether a human is in control of an account.
Twitter may require the account holder to verify a phone number or email address, or complete a reCAPTCHA.
These challenges are simple for authentic account owners to solve, but could be difficult (or costly) for spammy or malicious account owners to complete.
"Accounts which fail to complete a challenge within a specified period of time may be suspended," said the report.
Twitter said it particularly focused on detecting fake accounts at sign-up during the six-month period.
The decrease in the number of anti-spam challenges enforced during the second half of 2018 is related to the targeted efforts to detect fake accounts at sign-up and deter unhealthy accounts at the time of account creation, Twitter said.
"We also launched an enhanced spam reporting flow in October 2018 to provide people who use Twitter with the option to tell us more about the type of spam they experienced when they make a report," it added.
"Aggregate reports of spam have also decreased in the second half of 2018, suggesting that people continue to experience fewer spammy interactions on Twitter," said the report.
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