APA Comments on Homeland Security Advisory System

April 26, 2002

The Homeland Security Advisory System categorizes risk into five levels: Low, Guarded, Elevated, High and Severe.  Empirical research and theoretical concepts from behavioral decision research can be applied to understand the system’s strengths and weaknesses, as well as suggest and evaluate alternative designs. 

Decision researchers analyze risk in terms of two components: probability of event and severity of consequences.  The risk of a meltdown at a nuclear power plant has a very low probability, but very severe consequences.  In contrast, the risk of a train crash typically has a higher probability, but less severe consequences (i.e., fewer people are affected).

The HSAS is designed to communicate both of these components. The Presidential Directive that led to the system states, “Risk includes both the probability of an attack occurring and its potential gravity.”  The challenge of achieving this goal is twofold.  On the one hand, complex real-world situations need to be translated into the terms of the advisory system, so that each level consistently reflects a given degree of risk.  On the other hand, the users of the system must understand the intended risk level and translate it into personally relevant terms.

Behavioral decision research (the psychological study of decision making) suggests several specific versions of these challenges, facing the proposed system.  Each concern is grounded in existing research and could be clarified with existing theory and methods.

People interpret verbal descriptions of risk inconsistently. Different people interpret terms like “low,” “high,” and “severe” differently.  The same person may interpret a single term differently in different situations.  The ambiguity of verbal quantifiers poses a challenge to government officials who must categorize diverse, complex situations in a consistent way.  It poses a challenge to the citizens who must know what those alert levels mean (e.g., will “guarded” mean the same thing to different people?). 

Colors carry natural connotations.   The use of colors for risk categories must respect or override those norms.  The intention of the system is for green to correspond to the lowest risk and blue to correspond to one level higher.  This ranking fits that use of green in traffic signals.  However, it conflicts with the color spectrum, where blue is more extreme than green.   Although people may learn what HSAS means, under stress, people tend to “regress” to previously learned behavior, hence forget what they have learned.

Over time, people can learn how to interpret any consistently used scheme. Their willingness to invest in that learning will depend on the value that they see in the system.  HSAS promises five levels of risk, which could convey a lot of information and justify different behaviors.  However, if OHS sees our situation as a sustained crisis, then it might only use three levels, undermining the scale’s usefulness and credibility.  It might, then, be more practical and informative to use fewer levels to begin with.  That choice should be informed by an understanding of how people will respond to sustained demands (as studied in the literature on vigilance).

The distinction between probability and consequences is not a natural one for many people - even though it is easy enough to explain.  It may be worthwhile defining the scale so that its five levels communicate these two pieces of information.  For example, one might use the following interpretations, which should convey more information without compromising security: 

Level 1 (Low) corresponds to the situation where there is a low probability of a low severity event.

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