Intruder alert. The example is of a system designed for fence
intruder detection, consisting of infrared cameras watching the fences and a
control center. The cameras were capable of motion
detection and of estimating the range to the moving objects. The computer at
the control center sounded on each event of motion
detection. The system operation was verified through usability tests (e.g.
Dumas & Redish, 1999). However, in repeated
exercises, a vigilance problem was reported. The reason was high rate of false
alarms due to animals, birds and vehicles passing by the fences, and to bushes moving
in the wind (cf. Casey, 1998 – Never Cry Wolf).
The alert
system was redesigned according to usability guidelines for eliminating false
alarms by characteristics of potential intruders.