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Greg Mankiw weighs in on the Obama administration's economic stimulus measures in the Summer 2010 issue of National Affairs . And Mankiw cautions us to look at the macroeconomist models used to determine the state of the economy back at the beginning of 2009, rather than just the measures used to fight economic crisis. Mankiw writes that the Obama administration saw "decline in aggregate demand" as the key problem. Mankiw: So, inspired by the view that fiscal policy can prop up aggregate demand, Obama's advisors (and their congressional allies) began to design a stimulus plan heavy on direct government spending. A few days before President Obama's inauguration, his economic advisors released a document titled "The Job Impact of the American Recovery and Reinvestment Plan," in which they detailed some of their economic assumptions. They determined that the "government-purchases multiplier" — that is, the multiplier for direct spending — would be 1.57, while the tax-cut multiplier would be 0.99. In other words, every dollar spent by the government would yield $1.57 in aggregate demand, while every dollar in reduced taxes would yield only 99 cents in increased demand. And because 1.57 is larger than 0.99, the Obama team concluded it was better to increase spending than to cut taxes. Obama and his advisors arrived at these numbers through a standard macroeconometric model of the sort economists have been using for years. Such models take various past relationships among economic variables (inflation and unemployment, for instance) and extrapolate them into the future. In essence, the economy is modeled as a system of equations, each describing how one variable responds to many others. University of Chicago economist (and Nobel laureate) Robert Lucas famously criticized these models for lacking an appreciation of people's changing expectations; many economists, however, still find such models valuable, and have continued to employ them for forecasting and policy analysis. The question for economists now is whether the administration's assumptions, and the model based on them, were correct. After all, if we could be sure their model was right, we would know what to conclude when their stimulus plan was followed by 10% unemployment: The patient was sicker than they thought, and unemployment would surely have been higher still if not for the stimulus. (Indeed, since Obama's advisors do believe their model was right, this is the conclusion they have reached.) The trouble is, we have no way of knowing for sure if the model was in fact correct. To react to a model's failure to predict events accurately by insisting that the model was nonetheless right — as Obama's economic advisors have done — is hardly the most obvious course. Careful economists should instead respond with humility. When their predictions fail — as they often do — they should not dig in their heels, but should instead be willing to go back to their starting assumptions and question their validity. Read Crisis Economics here .