DHS's mission set is vast. It ranges from interdicting illegal immigrants, countering drug smuggling, marine safety, infrastructure protection, cyber security, disaster recovery, airport security, protecting the President, etc.
The broadness of this mission poses some interesting challenges for homeland security risk analysis. Risk analysis is performed in a variety of ways within the Department.... ranging from the tactical-level, where DHS agencies like the Office of Infrastructure Protection perform vulnerability assessments at critical infrastructure, to the strategic-level, where DHS headquarters tries to identify the most important homeland security risks facing the Nation in order to help set planning and budgeting priorities.
Things get especially interesting at the strategic-level. Through its annual budgeting process, DHS must look at its various mission sets and identify which ones should receive more or less funding. Homeland security funds should flow to those programs that have the greatest potential for total risk reduction. Theoretically, a strategic-level risk assessment should support that budgeting process along with various other inputs.
However, conducting a risk assessment that looks across natural disasters, manmade accidents, terrorism, and criminal activities (e.g., drug smuggling) is not easy. Good actuarial data exists for accidents and natural disasters (although global warming trends is beginning to make this data less reliable). We cannot depend on actuarial or historical data for terrorism issues, since terrorists are a dynamic and adaptive adversary. The best we can typically do with terrorism risks is calculate their relative likelihoods, as opposed to their absolute frequencies. But then , how do we compare terrorism and natural hazards risks if we can't put them on the scales?
One approach to comparing natural hazards and terrorism risks is doing "what-if" analyses. The analyst can prepare materials for the decision-maker with several "what-if" scenarios. He/she could prepare graphics that show how terrorism and natural hazards risks compare if the decision-maker believes that there will be a terrorist attack in the next 2 years vs 5 years vs 10 years vs 20 years.