Hats off to Vietnam's helmet law.

SCIENCE sciencemag.org significance that each can reduce cases and deaths. But efficacy in a carefully managed, tightly monitored study does not equal effectiveness in the messiness of the real world. Confusing matters further still, weather patterns, an economic upswing, or improved housing can also have a big impact on disease. The CGD researchers are part of a growing movement that seeks harder data about the number of lives actually saved by the billions poured into health in poor and middle-income countries. Such evidence is critical, proponents argue. After a decadelong explosion, funding for global health has leveled off (see p. 1258); governments and charities need to know the impact of their dollars to justify their investments and to change programs that don’t work well enough or not at all. The new field of what is called impact evaluation is rapidly gathering steam. Large global health donors and developing world governments have widely accepted that they need better evidence of what works, and several new institutes are devoted to gathering it. Publications that use impact evaluation methods have skyrocketed. It’s becoming increasingly difficult for the development assistance world to take credit for changes that might have occurred without their interventions— and to ignore the possibility that the money might have spared more people from disease if spent elsewhere. “Agencies have come to realize that impact evaluation is the only way you can meaningfully talk about results,” says Howard White, who heads the International Initiative for Impact Evaluation (3ie), a nonprofit launched in 2008. “They want to be able to go back to their funders or boards and say, ‘We’ve lifted 18 million out of poverty.’ ” But debates are raging about what constitutes convincing evidence of effectiveness. Randomistas, as some derisively call them, will only seriously consider supersized versions of the randomized, controlled studies used to evaluate the efficacy of drugs and vaccines. Others, like the researchers at CGD who select case studies for Millions Saved, considered other evidence as well. (The sidebars about By Martin Enserink