http://www.cs.uiowa.edu/~jones/voting/cogel/
Jones on Human Factors, Slide 6


 

Overvote rates for ES&S 150/550 users


Why the huge variation? Ballot layout is a big factor!
12 of these 14 counties used a 2-column layout for the presidential race!
Lake County - used a 1-column layout!
Levy County - ballot layout is unknown.

Overvote rate in Lake County is very similar to that in Columbia and Escambia Counties, using precinct-count systems without overvote protection.

 

Overvote rates for two-column ballots


Over 275,000 voters used ES&S central count ballots.
Of these, almost 92,000 were in Lake County,
    where about 3.4% cast overvotes.
Of these, over 172,000 used 2-column ballots,
    where about 6.1% cast overvotes.

In sum, ballot design problems may explain the loss of about 4700 presidential votes.