Fluctuations in investment are one of the most important sources of changes in aggregate expenditure. From the Web site of the Federal Reserve Bank of St. Louis or some other source, fi nd a data series on gross private domestic investment from 1990 to the present. Graph the annual percentage change in the series, and identify the periods when the economy was in recession. What can you say about the severity of the recession that started in December 2007 compared with the recessions of 1990–1991 and 2001?