Direct-recording electronic voting systems from several major vendors, including Diebold, ES&S, and Sequoia, have touch-sensitive screens. 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.
If a voter finds that they press the button for one candidate and a different candidate is highlighted, they have two options. First, while the calibration might be wrong, it is typically wrong the same way every time. A voter can experiment with selections, aiming “low” or “high” to see what it takes to get the proper button to be selected. Once a voter figures this out, they can then correct their previous mistakes. Alternately, a voter may ask for assistance from a poll worker. In any case, if the summary screen or paper printout (in states that have them) reflects any disagreement with the voter’s intent, then the voter should go back and correct the errors (with or without poll worker assistance).
If the poll worker determines that the voting machine is miscalibrated, then they will most likely need to “cancel” the ballot in progress. The poll worker can then place the machine in an administrative mode, recalibrate the screen, and allow the voter to start over again. This process should be safe to perform, even while the election is ongoing, without fear of losing votes stored in the machine.
In the long term, the only solution is for the voting system designers to use larger on-screen buttons, leaving more room for calibration error. Alternatively, some vendors avoid the calibration problem entirely by using hardware buttons instead of a touchscreen.