People who fill out so-called bubble forms—known to anyone who’s taken a standardized test—use distinctive pencil strokes that can later be used to identify them, Princeton University computer scientists have found. The techniques they developed could be used to catch cheaters on tests, the researchers say, but also for less benign purposes.
The researchers programmed a computer to take stock of 804 characteristics of a filled-in bubble. These include the mark’s center of mass, the variance of pencil-strokes from the bubble’s radius, the depth of shading in various regions of the bubble, plus more mathematically advanced measures.
Using 92 student surveys, the researchers then put the system to the test, After analyzing a dozen marks from each respondent, the researchers scrutinized 8 marks from one randomly selected respondent. The computer got its man (or woman) 51% of the time. And the correct answer was among the computer’s top 3 choices 75% of the time (and among the top ten 92.4% of the time).
The method could be used to catch students who hire proxies to take their SATs and teachers who change answers on their students’ high-stakes tests. (It might shed light on this controversy involving test scores in D.C. public schools, for example.) Bubble forms are also used in many voting districts, and the new findings could be deployed to detect fraud. On the other hand, the researchers said, employers might use the techniques described in the new paper to monitor the voting habits of their employees. (In the interests of transparency, some jurisdictions release scans of voters’ bubble forms, sans names.) Also, some anonymous surveys on sensitive subjects may be less anonymous than researchers or survey respondents think.
The researchers excluded forms that had been filled in using pens, but they suggest that it would be even easier to “de-anonymize” forms completed that way, because people using pens pay less attention to filling in the whole bubble.
Source: “Bubble Trouble: Off-Line De-Anonymization of Bubble Forms,” Joseph A. Calandrino, William Clarkson and Edward W. Felten (to be presented at the USENIX Security Symposium, August)