Since many states do not require people to register their guns, the number of Americans who actually own firearms cannot be precisely gauged. Some states provide estimates on gun ownership based on telephone surveys and, where possible, Miller and his colleagues used them. But they also used what statisticians call proxies-that is, they analyzed indirect measures of gun ownership. One, called Cook's index (named after Duke University economist Philip Cook) extrapolates the number of firearms in a state by averaging the percentage of suicides and homicides that are committed with firearms. A second extrapolates solely from the percentage of suicides that were committed with a firearm. "These proxies aren't reliable," says the NRA's Blackman. Cook's index, he says, is so inaccurate that "not even Cook accepts it anymore." And the telephone surveys were conducted on people over 18-an unreliable measure, he argues, of how many guns are available to kids. What really needs to be measured "is the percentage of households with guns."