I stumbled across this blog article, “County voter rolls may be inflated: Clerk says at least 50,000 may no longer live here.” The county in question is Washtenaw County, in Michigan.

The story quotes the Washtenaw County Clerk, Larry Kestenbaum:

Kestenbaum believes the number of people on Washtenaw County’s voting rolls is inflated by as much as 20 percent because they may not reflect voters who have moved away.

…

The result is that people who relocate may stay on voter rolls long after they’ve left the area. In communities with a high degree of transience, those names can add up.

“My guess is that at least 50,000 of the names we show as registered voters are people who no longer live here,” Kestenbaum said.

There are 254,625 registered voters on the books in Washtenaw County; 92,000 of them in the city of Ann Arbor.

Later in the story, Ann Arbor City Clerk Jackie Beaudry discussed on method used to check on the accuracy of their registry:

The only cleanup comes when local clerks have a reason to contact voters.

“When we send information to voters, we collect the names from mail that’s returned as undeliverable,” said Beaudry. The City Clerk’s office then sends a follow-up letter asking the voter (or former voter) to confirm or update their address.

“Some do respond and tell us they’ve moved or that they’re away for a few months, or a year, but intend to come back and are still local voters,” she said.

Those who fail to respond to the second communication are put on a “countdown” list. Those on the list who fail to vote in two even-year general elections are purged from the voter file, said Beaudry. “It’s a long process.”

When a downtown polling place was moved from the former YMCA property to the Ann Arbor District Library, for instance, many outdated names were found, Beaudry said.

The cost of a regular, systematic check is prohibitive.