The Guidelines [NPG] establish a risk-based approach to preparedness. Risk is a function of three variables: threat, vulnerability, and consequence. Both threat and vulnerability are influenced by the probabilities of events that are highly uncertain. In order to compensate for that uncertainty, the Guidelines provide a set of National planning Scenarios that represent a range of threats that warrant national attention. The National Planning Scenarios establish common assumptions to guide planning nationwide regarding potential vulnerabilities and consequences (or impacts) of major incidents. Analysis of the range of potential impacts is essential for defining capabilities in terms of both capacity (i.e., how many are needed) and proficiency (i.e., how well must they be able to perform). These capabilities must be reflected in emergency operations plans (for the near-term) and in preparedness strategies (for the longterm). Federal, State, local, tribal, and territorial officials supplement this approach with risk assessments that provide additional data on their specific threats, vulnerabilities, and consequences. As a result, officials can tailor their approach according to differences in risk across the Nation. (DHS, National Preparedness Guidelines, September 13, 2007, p. 3)