DNA Identification in Mass Fatality Incidents
Open vs. Closed Incidents
Mass fatality incidents have been described as either an “open” system or a “closed” system. Open systems are those in which the number and identity of victims is unknown. Closed systems are those in which the number and identity of victims is more certain, such as in a small car or airplane crash.
As noted in chapter 4, Major Decisions, a closed system can become an open system. Likewise, an open system can approach a closed system—or even become a closed system—as a larger proportion of RMs are identified. This concept proved useful in the WTC identification effort. As the number of victims was reduced from the initial estimate of 5,000 to approximately 2,750, the prior probability was accordingly increased, resulting in the KADAP’s recommendation that the likelihood ratio necessary to make a DNA-based identification be reduced. [Note: Gender was another factor used to modify the prior probability.]
On the other hand, in a closed system, prior probability is increased as victims are identified, reducing the likelihood ratio necessary to meet the statistical threshold for identification. In rare instances (for example, in a small closed system in which the DNA profiles of all of the victims are known), it may not be necessary to establish a statistical threshold in order to make a DNA identification. Needless to say, DNA-based identifications are more straightforward in a closed system than in an open system.
Although open-system incidents may require collection of family reference samples on a much wider scale, the general principles for DNA identification are the same. (See chapter 10, Sample Tracking and Management, and chapter 11, Sample Analysis, for more detailed discussions of sample issues.)
