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E-voting usability: Touchscreen problems in NM, TX and FL

elections

Ed Felten notes a reccuring problem in early voting in a number of states (Texas, New Mexico and Florida) where people think they chose one candidate but the ballot review screen shows the machine registered a vote for another candidate. Felten goes on to take a shot at figuring out what the source of the problem is:

My guess is that the touchscreens are miscalibrated. Touchscreens use one mechanism to paint images onto the screen, and a separate mechanism to measure where the screen has been touched. Usually the touch sensor has to be calibrated to make sure that the coordinate system used by the touch sensor matches up with the coordinate system used by the screen-painting mechanism. If the sensor isn't properly calibrated, touches made on one part of the image will be registered elsewhere. For example, touches might be registered an inch or two below the place they really occur.

Actually, this is more than likely not a mis-calibration of the touchscreen.

As Doug Jones noted in his recommendations to Miami-Dade county (PDF), frequently voters will rest their hand on part of the touchscreen monitor and occasionally, a thumb or part of the palm will fall on the touchscreen surface. This will shift the calibration grid of the touchscreen towards the errant digit. So, when a voter presses on their choice for the vote, it might be registered by the machines as a choice for another candidate. Here's a great illustration of this effect from page 22 of Doug's report:

illustration of how an errant digit can throw off touchscreen calibration

What's the moral of this story? Make sure that every voter actually critically examines the ballot review screen at the end of their voting session. If you'll be a TechWatch volunteer, poll worker or call center operator, make sure that you stress to everyone that examining the review screen on paperless DREs is very very important.

If you live in California, you have a choice: paper or plastic?

UPDATE: Note that this property has been used by Ping to produce a two-handed touchscreen interface. (Ping is about a billion times smarter than most people I know.)