helping to identify a suspect. Such uncertainty is aggravated by the fact that p (let alone separate x, y, and z values) in any real-world case would likely be unknown. The estimates above assume that all of the incorrectly coded particles bear the same signature; if they were to differ, the opportunity for misinterpretation would be diminished, and if necessary, more elaborate combinatorial formulas could be invoked to provide quantitative estimates. Likewise, recovery of substantially more than 10 taggant particles would obviously increase the "signal-to-noise" ratio and would improve the ability to use this evidence in criminal proceedings.

  1. The contamination attributable to x (manufacture) and y (transportation and distribution), and to a lesser extent z (environment), would depend on the frequency with which the tagging code was changed. If the intervals were long (e.g., 6 months, as in the Swiss protocol), contamination would be insignificant except during manufacturing lot changes. Of course, such infrequent changing of the code implies reduced information content for the taggants.

  2. As a rough average, 1 pound of an explosive is required to dislodge 1,000 pounds of rock, mineral, or ore in mining operations. However, circumstances (surface mining versus tunnel mining, and the materials involved) can cause this ratio to vary somewhat. Each pound of explosive might be tagged at a level of approximately 10,000 particles per pound. Perhaps 1 in 1,000 of these can be expected to survive the mining blast in readable form, to be transported away from the mine. Depending on the subsequent processing steps used to produce usable construction material (such as wallboard, concrete, etc.), the concentration of taggants might be decreased or increased. In assessing the resulting environmental contamination that such building materials might produce when directly involved at a criminal bombing event, it is estimated that about 1 taggant particle per 100 pounds of material could be anticipated. Clearly, though, a substantial uncertainty applies to this estimate.

  3. The least equivocal message that emerges from these considerations is that more real-world data need to be accumulated to tighten each of the contamination estimates and their quantitative implications.

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