"We propose and demonstrate a simple method for guiding researchers in developing quantitative models of economic fluctuations. We show that a large class of models are equivalent to a prototype growth model with time-varying wedges that resemble time-varying productivity, labor taxes, and capital income taxes. We use data to measure these wedges, called efficiency, labor, and investment wedges, and then feed their measured values back into the model. We assess the fraction of fluctuations in output, employment, and investment accounted for by these wedges during the Great Depression and the 1982 recession. For the Depression, the efficiency and labor wedges together account for essentially all of the fluctuations; investment wedges play no role. For the recession, the efficiency wedge plays the most important role; the other two, minor roles. These results are not sensitive to alternative measures of capital utilization or alternative labor supply elasticities. We argue that these results suggest that standard models of credit market frictions are unpromising avenues for business cycle fluctuations"--Federal Reserve Bank of Minneapolis web site.