Professor John FitzGerald:

If one just asks what happens if house prices fall by 30%, one will get an effect. It becomes much worse if unemployment goes through the roof and Government revenue collapses with the Government then having to tax the hell out of the economy. That is a far worse scenario. What they were doing was just picking a housing crisis and unemployment independently, the reason being they did not have a model. To do the scenario properly in a way that is internally consistent, one needs a model of the economy. The only people with a model of the economy were in the ESRI.