The probability that an intrusion detection sensor will sense an
unauthorized action (Garcia 298).

The probability of sensing (PS)
is the probability that a sensor will alarm for a particular type
of intrusion under certain conditions. The PS value is a measured value and should be
associated with a statistical confidence level. The design of
detection systems should take into account certain factors that
affect P_{S} such as environmental conditions (e.g., fog,
rain, snow, etc.), installation conditions, and the condition of
the equipment (a sensor's P_{S} value may degrade over
time). In addition, the design needs to account for the type of
action by an intruder to be detected (e.g., running, walking,
jumping, crawling, tunneling, breaching, etc.).