Congressional Budget Office Director Douglas Elmendorf stalled health care reform recently by questioning the fiscal impact of Congressional proposals already far down the legislative road. The CBOs perceived authority and expertise reinforces the impact of such pronouncements. But if history is a guide, this emperor has no clothes.Put most simply, the CBOs track record in predicting the effects of health legislation is abysmal. Over the last two decades, the CBO has routinely overestimated the costs of expanded government health care benefits and underestimated the savings from program changes designed to reduce expenditures. Most recently, it overestimated the five-year cost of Medicare Part D the prescription drug benefit - by more than 35%. Even more dramatically, the CBOs estimates of the Medicare savings from the Balanced Budget Act of 1997 underestimated the impact, on average, by a full 100%. Thats right: In the BBAs first three years, Medicare spending fell fully twice as fast as the CBO had projected.These mistakes arise neither from a hidden partisan agenda nor a shortage of competence or commitment. The CBOs reputation for scrupulousness, thoroughness, and nonpartisanship is well-deserved. But the very processes in which it is asked to engage, and the ways in which its results are used, make serious misjudgments almost inevitable.To start, health care is very complicated, and even with all the economists, investors and policy wonks who study it, its financial dynamics are never fully defined. Government policies certainly influence health care costs, but, as the CBO acknowledges, so do technological and labor-market changes, fallout effects from the larger economy, and mass psychology among health care providers and insurers, all of which defy predictive models. Further, the economists who dominate the CBO assume that health care providers will respond to changing economic incentives as though they were rational, profit-maximizing firms, but hospitals, managed care plans and large group practices are complex organizations with multiple, competing objectives. In the short run, they often do exactly the opposite of what microeconomic theory might predict. Perhaps most importantly, the CBO is now routinely expected to project the impacts of policy changes over 10 years, an absurdly long time to predict what will happen in so large and complex a sector as health care. Fortunately, another flaw in 10-year projections offers the way out of CBO-induced paralysis. The CBOs most basic forecasting assumption is ceteris paribus: nothing else, other than the policy it is evaluating, will change during the forecast period. Technically, its hard to imagine forecasting any other way, but everything always does change, most notably policy itself. Ten years gives us a lot of time to fix mistakes.