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2 Elements of Detection: Concepts, Threats, and Devices
Pages 18-34

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From page 18...
... SENSITIVITY, SPECIFICITY, AND ROC CURVES The basic statistical and probabilistic elements of assessing performance of any detection methodology include sensitivity, specificity, and receiver operating characteristic (ROC) curves.
From page 19...
... . We typically treat the detector response as a random variable and are interested in the following conditional probabilities (letting D denote the status of the detector and T denote the status of the true presence or absence)
From page 20...
... medical diagnosis and signal processing. The ROC curve is defined to be a plot of sensitivity versus
From page 21...
... Figure 2.3 illustrates the ROC curve associated with the simple example from Figures 2.1 and 2.2. ROC curves by definition begin at the point (0,0)
From page 22...
... . ROC curves provide a means to compare two competing detection techniques.
From page 23...
... FIELD PERFORMANCE OF DETECTORS Sensitivity, specificity (equivalently true or false positive rates) , and ROC curves reflect commonly reported performance measures for any proposed detector, but one should keep in mind that even a detector with strong sensitivity and specificity can have unacceptable performance in the field, where a performance probability of interest is Pr(T = Yes|D = Yes)
From page 24...
... / (a + b + c + d) from the assessments of the test reflects the experimental design and not the true marginal probability of explosive presence among screened objects or locations in the field.
From page 25...
... For example, consider the performance of airport metal detectors as "weapons detectors." Most alarms occurring during screening are false with respect to weapons (i.e., the detectors detect metal, but not weapons) , but most passengers accept such false alarms since there are many non-weapon reasons for an alarm to go off.
From page 26...
... In this application, system effectiveness is defined as "a measure of the degree to which an item can be expected to achieve a set of specific mission requirements and which may be expressed as a function of availability, dependability and capability."1 For standoff explosives detection, system effectiveness could include detection system availability, maintainability, system ROC, and the probability of avoiding defeat. THREAT SCENARIOS In considering any situation involving standoff detection of explosives, one must have some general understanding both of the overall scenario and of the parameters that describe the explosive device and the surrounding environment.
From page 27...
... A closed situation is one in which public access to the location or event is limited, and all persons present must pass through certain checkpoints before being admitted.2 An example would be a Super Bowl, where all attending persons enter via stadium gates, or a military base, with a limited number of entrances manned by military security personnel. In such situations, the entry of HE into the area can be controlled in principle through applying appropriate detection technologies at the controlled checkpoints.
From page 28...
... A bombing in a crowded sports stadium will have a much greater impact than a bombing in an empty sports stadium, whereas blowing up a dam will likely have the same impact regardless of when the explosion occurs. Some agencies and organizations interested in standoff explosives detection have specific areas, facilities, or people that they are tasked with protecting, while others (e.g., police)
From page 29...
... Humidity can be a factor, both because some explosive vapors may form clusters with water molecules and because interactions of the probing radiation with water vapor in air can limit the application of techniques such as terahertz spectroscopy. Dust particles in air can adsorb explosive vapors, interfering with some types of vapor detection but possibly also leading to improved sampling strategies based on collection of the particles.
From page 30...
... Threat Parameters That Describe the Device Several key threat parameters characterize any explosive device. These include the type of explosive used, the mass of explosive, and several other factors relating to the device construction.
From page 31...
... Besides the explosive material, other important device components include the casing or shielding around the main charge, the detonator, and any associated wiring. Very often, some or all of these components will be made of metal, and metal detection thus becomes one possible means of detecting the explosive device in some circumstances.
From page 32...
... An explosive detonation can cause additional harm to both humans and the environment if the detonation is used to disperse radioactive materials or biological or chemical agents. Scenarios involving such dispersal are outside the scope of the present study.
From page 33...
... ORTHOGONALITY Two or more explosive detection technologies are completely orthogonal if the detection methods are mutually independent. That is, they detect independent characteristics of the explosive device.
From page 34...
... However, the use of orthogonal detection technologies presents a significant challenge to implementation. That challenge is how to process the information from two orthogonal technologies when one is indicating positive for the presence of an explosive device and the other negative.


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