To some extent, the methods used to set enlistment standards have been influenced by the type of characteristic under consideration. Many medical and physical standards are determined by the presence of disqualifying conditions that are justified on prima facie grounds: examples are serious diseases, physiological abnormalities, and such physical impairments as blindness or deafness. For these types of serious medical or physical conditions, a formal validation methodology is usually unnecessary.

More formal, quantitative methods have been developed for validating education and mental aptitudes for two reasons (National Research Council, 1988). First, a characteristic like aptitude is a continuum rather than a present/absent dichotomy; one therefore has to develop cutoff scores to determine eligibility. Second, while education can be considered a dichotomous characteristic (high school graduate versus nongraduate), being a nongraduate is not disqualifying on its face in the same way that blindness might be. Rather, nongraduates (and lower aptitude recruits) can reduce military effectiveness in various ways, such as having high attrition rates or poor performance with respect to certain military duties. Thus, there is a trade-off between effectiveness or performance and the proportion of recruits with these characteristics.

Although formal trade-off models have not been applied to the validation of medical and physical standards, they may be useful when medical or physical attributes are similar to education and aptitude. For example, they could be applied to continuous conditions, such as weight and strength, for which no disqualifying level can be established clinically, or to dichotomous conditions, such as marijuana use or asthma, the presence of which may reduce effectiveness but is not automatically disqualifying.

Elements of a Formal Model

Generally, in order to develop a formal trade-off model, there must be a correlation between a selection characteristic and some desired or necessary outcome. If there is no correlation, then the validation process basically ends with no need for a selection standard, since the characteristic is not related to a valued outcome. At the other extreme, if every person with a certain characteristic at entry has an undesired outcome, then there is no need for a trade-off analysis, since everyone with that characteristic should be disqualified. In effect, for those medical and physical conditions that lead to automatic disqualification, it is assumed that all recruits with those characteristics are unable to perform any military specialty at a

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