Daily Archives: October 27, 2010

National: Vote Flipping and Touch Screen Calibration

Again this election cycle, stories have emerged about “vote flipping”, most notably in Texas, where a video of erratic touchscreen behavior was posted on several sites, and in several North Carolina counties. (link, link, link, link) As voting technology expert Douglas Jones wrote several years ago, it seems unlikely that vote flipping is evidence of intentional hacking. However, these incidents do highlight the lack of transparency of software-generated election results and undermine confidence in elections generally. Vote flipping can be caused by a voter touching the screen in two places, for example resting one hand on the machine while making selections with the other (see pp. 20-22 here), but the most likely cause of “vote-flipping” is miscalibration. As Rice University computer scientist Dan Wallach explains in a post at ACCURATE:

The screen shows pictures of buttons with labels for the various candidates, which the voter selects by touching the screen with their finger. Some voters using these machines have reported problems where they pressed the button for one candidate and a different candidate was selected. These issues are most likely the result of poor touchscreen calibration rather than any security problems with the voting machines’ software.

The clear, touch-sensitive layer is separate from the part of the screen that displays the buttons. The thickness of the touch-sensitive layer directly implies that when different voters are looking at the screen from different angles, they will naturally want to touch the screen at different locations. This can be partly addressed by “calibrating” the touchscreen in advance. The calibration process, familiar to anyone who owns a PDA, involves the machine displaying a series of cross-hairs and asking the user to press on the center of each cross-hair. The machine then computes a correction to ensure that selections are mapped to the correct part of the screen below. Of course, if the calibration was done incorrectly, or even if the voter is notably taller or shorter than the person who did the calibration, then presses on the screen might still be misinterpreted. Furthermore, different voters may use different parts of their finger (ranging from the fingernail to the whole finger), which may differ from how the system was calibrated. (See also “Touch Screen Usability: Election Edition!” and “Vote Flipping and Touchscreens“) Vote flipping was investigated in several articles during the 2008 election cycle. Computerworld interviewed both voting machine vendor and election integrity activists for “Are design issues to blame for vote ‘flipping’ in touch-screen machines?” and Wired magazine posted an article about the potential for maliscious calibration as detailed in the Ohio EVEREST report.