“inference,” where the goal is to infer something about the unobserved population based on the data obtained from the observed sample. Thus it is correct to say that all such tests, including the Army’s original body armor test protocol, are statistically based. However, it is critical to note that not all statistically based tests are statistically principled.

A “statistically principled” test uses appropriate statistical methods to properly make formal inferences about the population from the sample. Formal inference means that the desired characteristic or characteristics in the population are estimated from the sample data in such a way that uncertainty inherent in the inference from sample to population is appropriately and explicitly accounted for by the statistical methods. In the case of testing, this generally means a particular sample size is specified (as well as other sampling and estimation details) to minimize the uncertainty to some acceptable level. Thus, the use of statistically principled test procedures and test methods allow decision makers, test organizations, and manufacturers to all have confidence that the test performance of the sample appropriately characterizes the performance of the population.

Uncertainty and Variation Drive Overdesign

Larger and/or thicker body armor insert plates provide additional survivability but at the cost of more weight. Heavier body armor can contribute to fatigue, may inhibit mobility and effectiveness, and, at its worst, may result in personnel choosing not to wear the body armor, completely defeating its purpose (OTA, 1992).

Body armor is designed to protect against a particular level of threat. To the extent that the armor exceeds this level, it can be thought of as overdesigned or overmanufactured, in the sense that lighter plates could have been produced to achieve the desired level of protection.

Uncertainty and variation in the manufacture, testing, and employment of body armor, as well as the natural concern for protecting personnel, tend to result in conservative decision making, which in turn can result in body armor overdesign and/or overmanufacture. For example,

•  Variation in body armor manufacturing processes can drive suppliers to produce plates that are generally heavier than required to lower the risk of producing nonconforming plates.

•  Variation in FAT and LAT can further drive suppliers to produce heavier-than-necessary body armor to ensure their product successfully meets the FAT and LAT test standards.

•  Uncertainty about the particular threat that personnel may face can result in tighter specifications and/or testing to a higher possible threat and sometimes to threats beyond what personnel would actually experience in order to ensure that the threats are clearly met.



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