A House of Commons Science and Technology Committee paper from 2011 Scientific advice and evidence in emergencies , Third Report of Session 2010–11 concludes that it “had misgivings about the Government’s communication of what it termed ‘reasonable worst case scenarios’, that is, the worst situation that might reasonably happen”.
The committee believed that “While such scenarios are useful for organisations preparing for, and responding to, emergencies, use of such scenarios led to sensationalised media reporting about the projected deaths from swine flu. We concluded that the Government must establish the concept of ‘most probable scenarios’ with the public, in all future emergencies”.
It also reported (para 82) that for bird flu it chose not to take the fatality rate from the bird flu of 60% but the 2% from Spanish Flu as the worst number gave a situation that was “almost unpreparable for” leading to (para 87) “We are concerned that the word “reasonable” appears to be influenced by the need to find a reasonable level of public expenditure for contingency planning rather than outlining the worst scenario that might realistically happen, based on the best available evidence“.
So, Worst Reasonable Case is not good for public communication and not always used as the basis for planning assumptions.