If bubble marking patterns were completely random, a classifier could do no better than randomly guessing a test set’s creator, with an expected accuracy of 1/92 ≈ 1%. Our classifier achieves over 51% accuracy. The classifier is rarely far off: the correct answer falls in the classifier’s top three guesses 75% of the time (vs. 3% for random guessing) and its top ten guesses more than 92% of the time (vs. 11% for random guessing). We conducted a number of additional experiments exploring the information available from marked bubbles and potential uses of that information. See our paper for details.
via Freedom to Tinker – New Research Result: Bubble Forms Not So Anonymous. Even something as simple and low tech as bubble forms can’t be totally anonymous.