"I would say hundreds of thousands of eligible voters in Pennsylvania do not have a valid PennDot ID," Siskin testified.
Alicia Hickok, a lawyer representing the state, challenged his methodology, saying he had failed to count those with access to other types of valid ID, such as 835,000 students attending Pennsylvania colleges. She said he also might have counted some voters, such as those with similar names, more than once.
Siskin said his methodology was sound and his estimates conservative, and that even factoring in an error rate, the number of those without acceptable ID was still well into the six-figure range.
He was testifying on the second day of what is expected to be a two-week trial before Commonwealth Court Judge Bernard McGinley over the controversial 2012 law requiring voters to provide state-approved photo ID before voting.
The plaintiffs, who include the NAACP, the League of Women Voters, and other groups as well as several individuals, were granted an injunction last year, and the law was not enforced in the Nov. 6 presidential election or this year's May primary. Gov. Corbett and Republicans who support the law say it will prevent voter fraud; Democrats who oppose it say it will disenfranchise minorities, the elderly, young people, and the disabled.
Siskin said his research determined that twice as many African American and Hispanic voters did not possess valid photo ID, and therefore those groups were more significantly affected by the law.
Among almost 900,000 black registered voters, 89,000, or nearly 10 percent, do not have valid ID, compared with just over 5 percent of white Pennsylvania voters, Siskin said. He said the data showed 11 percent of Asian American residents did not have proper ID.
More Democrats than Republicans do not have forms of identification required by the law, he said, and fewer people between ages 18 and 22 or older than 70 have valid ID.
Siskin said the numbers are high enough to be statistically significant.
He was asked on the witness stand to respond to a report by statistician William Wecker, an expert expected to testify for the state, who challenged Siskin's figures. Wecker's report suggests a higher number of people than in Siskin's findings have access to acceptable alternative forms of ID - such as college students, members of the military, and nursing-home residents.
Siskin contended Wecker's methodology was flawed by overstating the number of college students with access to valid college IDs who were among all people residing near colleges.
Nils Frederiksen, a spokesman for the Corbett administration's Office of General Counsel, said Siskin was working with a narrow slice of data that was not accurate or valid.
"There is far more data out there," he said. "They are arguing that people are left out when they purposefully left them out, such as nearly one million college students who have access to IDs."
Frederiksen said that regardless of how many people don't have acceptable ID now, "100 percent of voters can get ID if they make one visit to PennDot."
Contact Amy Worden at 717-783-2584 or firstname.lastname@example.org or follow @inkyamy on Twitter.