A very reasonable process when confronting policy decisions is to consider the economic impact of those decisions. Recently, the suggestion of 20% unemployment has been repeated by media because Treasury Secretary Mnuchin used that number when briefing Congress. This was not an estimate based on any kind of model or thoughtful analysis as stated by the secretary himself. I am certain that in the next few weeks there will be numerous estimates of economic impact hopefully with better analysis but none of them will be adequate to support policy decisions.
Many firms have access to high-frequency data on credit card transactions from which they can estimate changes in spending. Most economists neither have access nor are familiar with these data sets. They will try to use examples from the Spanish Flu or from more minor events that have occurred in structurally different global economies through time.
One of the confounding factors in forecasts, is that the US has become accustomed to using markets as a signal for policy. While markets do provide useful information, they also incorporate changes in risk preferences and perception which are often confused with event probabilities. An accurate economic forecast must now factor in policy changes resulting from changes in risk perception which may or may not be related to actual events.
The answer to the economic impact question is that there will be heroic assumptions made on the effectiveness of unknown future health policy. Further heroic assumptions will estimate the interaction of unknown future monetary and fiscal policy with health policy. All of this will be based on the (much more likely) assumption that medical professionals will know more about how COVID-19 is transmitted and will development more effective treatments over time.
We should find comfort in the fact that the lack of certainty is not uncommon. When stakes are low, it is easy to fool ourselves into thinking that the world is certain. During times of crisis, we should remember that the world was never as certain as we thought, but that is OK.