"I'm sorry, Dave, you aren't allowed to cheat on your exam."
Caveon, an independent test security company, uses statistics to detect cheaters:
"...Caveon says its analysis of answer sheets is the most sophisticated to date. In addition to looking for copying, its computers... hunt for illogical patterns, like test-takers who did better on harder questions than easy ones. That can be a sign of advance knowledge of part of a test.Cheaters Find an Adversary in Technology, The New York Times>>
The computers also look for unusually large score gains from a previous test by a student or class. They also count the number of erasures on answer sheets, which in some cases can be evidence that teachers or administrators tampered with a test.
When the anomalies are highly unlikely — their random occurrence, for example, is greater than one in one million — Caveon flags the tests for further investigation by school administrators."