Resumen
It is informed that surveys in the U.S. have found that 95 percent of schools, 43 percent of public libraries and 33 percent of teenagers' parents employ filtering software to block access to pornography and other inappropriate content. Many products are also now available to filter out spam email. In 2000, the U.S. Congress passed the Child Internet Protection Act mandating that schools and libraries install content-filtering software in order to be eligible for some forms of federal funding. A district court struck down the requirement for libraries on the grounds that it violates the First Amendment. Much of that court's finding of facts was devoted to analyses of error rates and some of the arguments made on appeal to the U.S. Supreme Court also hinged on analyses of error rates. Most empirical studies of error rates have suffered from methodological flaws in sample selection, classification procedures, or implementation of blocking tests. Results have also been interpreted inappropriately, in part because there are two independent measures of over-blocking that are sometimes confused, and likewise for underlocking. This article presents a framework to guide the design and interpretation of evaluation studies. While the framework applies with only minor modifications to the evaluation of spam filters, the examples and discussion here focus on pornography filters.