The data banks of the National Bureau of Economic Research contain time-series
data on 2000 macroeconomic variables. Even if observations were available since
the birth of Christ, the degrees of freedom in a model explaining gross national
product in terms of all these variables would not turn positive for another two
decades. If annual observations were restricted to the 30-year period from 1950 to
1979, the degrees of freedom deficit would be 1970.