An optical answer sheet (or "bubble sheet") is a special type of form used in multiple choice question examinations. Optical mark recognition is used to detect answers. The most well known company in the United States involved with optical answer sheets is Scantron, although certain applications require their own customised system.
Optical answer sheets usually have a set of blank ovals or boxes that correspond to each question, usually on separate sheets of paper. Bar codes may mark the sheet for automatic processing, and each series of ovals filled will return a certain value when read. In this way students' ability can be tested, or identity given.
Most systems accommodate for human error in filling in ovals imprecisely, as long as they do not stray into the other ovals and the oval is almost completely filled.
It is possible for optical answer sheets to be printed incorrectly, such that all ovals will be read as filled. This occurs if the outline of the ovals is too thick, or is irregular. During the 2008 U.S. presidential election, this occurred with over 19,000 absentee ballots in the Georgia county of Gwinnett, and discovered after around 10,000 had already been returned. The slight difference was not apparent to the naked eye, and was not detected until a test run was made in late October. This required all ballots to be transferred to correctly-printed ones, by sequestered workers of the board of elections, under close observation by members of the Democratic and Republican (but not other) political parties, and county sheriff deputies. The transfer, by law, cannot occur until election day (November 4th).
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