Square root biased sampling

Square root biased sampling is a sampling method proposed by William H. Press for use in airport screenings, as a mathematically efficient compromise between simple random sampling and strong profiling.

Using this method, if a group is $n$ times as likely than the average to be a security risk, then persons from that group will be ${\sqrt {n}}$ times as likely to undergo additional screening. So, for example, if someone from a profiled group is nine times more likely than the average person to be a security risk, then when using square root biased sampling, people from the profiled group would be screened three times more often than the average person.

Development

Press, a professor in the field of computer sciences and computational biology, was looking for a way to sample long sequences of DNA. Press developed square root biased sampling to solve this problem. He later found that it had been developed independently by Ruben Abagyan, a professor at the Scripps Research Institute in La Jolla, California, for use in a different context. Press later proposed the use of square root biased sampling for use by airport security, in a paper published in Proceedings of the National Academy of Sciences.

He argued that this method would be a more efficient use of the limited resources possessed for screening, as compared to the current practice, which can lead to screening the same persons frequently and repeatedly. However, use of this method presupposes that those doing the screening have accurate statistical information on who is more likely to be a security risk, which is not necessarily the case.