"A scenario-based plan is developed from a National Planning Scenario; there are currently 15 National Planning Scenarios. These scenarios have been grouped into eight sets. National Planning Scenarios will be developed, updated, or amended as necessary at least every two years. This process will be informed by a risk-based analysis intended to focus planning efforts on the most likely or most dangerous threats to the homeland. Annex I to HSPD-8 specifically addresses this planning approach". (FEMA, Interim IPS (Draft 2.3), July 3, 2008 copy, p. 2-11)