False alarms

No portal metal detector is manufactured with the correct adjustments that meet all users' needs. These adjustments or settings are generally made by the vendor when the detector has been installed in the area where it will ultimately be operational. Given equivalent environments, however, different facilities have different requirements for equipment sensitivities. A metal detection program in the U.S. Treasury Department will have very different equipment settings than a program for a school weapon-detection portal. The optimal settings for each facility will be a set of tradeoffs that balance false-positive errors against false-negative errors.

A false-positive error occurs when an alarm occurs for an otherwise acceptable item, such as a metal key ring. These errors occur more frequently in a program that seeks to err on the side of security. False positives can be extremely annoying to scannees and can increase the manpower required to support a metal detection program. Constant false-positive alarms can also cause the operators of a system to become desensitized to alarms, so that they eventually fail to fully investigate the sources of all alarms.

A false-negative error occurs when no alarm is triggered by an unacceptable item, such as a weapon. These errors may occur more frequently in a program that seeks to err on the side of convenience. A system set more toward false negatives can slightly increase the risk of a weapon entering the facility but generally helps a metal detection program to run as smoothly and quickly as possible. In such a program, when an alarm does occur, the operators will be more likely to take it seriously and to investigate fully what caused the alarm. Many school system programs will be set in this manner.

Most portal metal detectors are additive; they will generate an alarm based on the total response received from the metal detected on a scannee. An alarm does not necessarily mean just one suspicious item has been detected. Because of this, a scannee who has multiple "borderline" items on his other body has a better chance of causing a false alarm. See exhibit 3.7 for a pictorial description.