A letter report issued by the Government Accountability Office with an abstract that begins "Data mining--a technique for extracting useful information from large volumes of data--is one type of analysis that the Department of Homeland Security (DHS) uses to help detect and prevent terrorist threats. While data-mining systems offer a number of promising benefits, their use also raises privacy concerns. GAO was asked to (1) assess DHS policies for evaluating the effectiveness and privacy protections of data-mining systems used for counterterrorism, (2) assess DHS agencies' efforts to evaluate the effectiveness and privacy protections of their data-mining systems, and (3) describe the challenges facing DHS in implementing an effective evaluation framework. To do so, GAO developed a systematic evaluation framework based on recommendations and best practices outlined by the National Research Council, industry practices, and prior GAO reports. GAO compared its evaluation framework to DHS's and three component agencies' policies and to six systems' practices, and interviewed agency officials about gaps in their evaluations and challenges.."