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TAMPA, Fla. - As mail-in voting surges in Florida, more ballots are also being thrown out.
In Hillsborough County, almost 228,000 people voted in last week's primary. Only 110 ballots cast on Tuesday in-person were thrown out, but mail-in voting proved more perilous.
More than three times that many mail ballots, 356, were rejected.
The most likely reason for a ballot being rejected is a mismatched signature, according to Hillsborough County elections supervisor Craig Latimer.
Of 356 thrown out, 211 were deemed to have a mismatched signature, while 145 weren't signed at all.
"We actually verify these signatures," said Latimer. "The people looking at these signatures have been trained. They know what they're looking at."
RELATED: How Florida mail-in ballots are processed at the elections office
Mail voting has seldom been more popular. In 2016's March presidential primary, 30% voted by mail. This year, it was 54%.
Dr. Daniel Smith at the University of Florida just finished a study that says people who are voting by mail for the first time tend to have ballots thrown out more often.
He says it tends to be younger voters, those not registered to a major party, and military dependents.
"Voters who are unfamiliar with voting by mail are more likely to make mistakes," said Smith.
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RELATED: Experts agree with politicians: Voting by mail in Florida is safe and secure
A report by National Public Radio said more than 500,000 presidential primary votes were thrown out nationwide.
In Florida, that number was 18,000; less than 1% of ballots cast, but more than enough to decide who wins the presidency.
"We have three really good methods of voting in Florida," said Smith. "People need to find their comfort zone. Do it early, if possible, and hope that we don't have a close election."
According to the Florida Division of Elections' website, a supervisor of elections is required to notify a voter "as soon as it is practical" if their signature does not match the signature on file, or if the signature is missing. For more information about resolving a missing or mismatched signature, visit https://dos.myflorida.com/elections/for-voters/voting/vote-by-mail/.