GAO found numerous problems in DOD's processes for recording and reporting costs for GWOT, raising significant concerns about the overall reliability of DOD's reported cost data. As a result, neither DOD nor Congress can reliably know how much the war is costing and details on how appropriated funds are being spent, or have historical data useful in considering future funding needs. On the basis of GAO's work, DOD is taking steps to improve its cost reporting. Factors affecting the reliability of DOD's reported costs include long-standing deficiencies in DOD's financial systems, the lack of a systematic process to ensure that data are correctly entered into those systems, inaccurately reported costs, and difficulties in properly categorizing costs. In at least one case, reported costs may be materially overstated. Specifically, DOD's reported obligations for mobilized Army reservists in fiscal year 2004 were based primarily on estimates rather than actual information and differed from related payroll information by as much as $2.1 billion, or 30 percent of the amount DOD reported in its cost report. In addition, GAO found inadvertent double counting in the Navy's and Marine Corps' portion of DOD's reported costs amounting to almost $1.8 billion from November 2004 through April 2005. Because it was not feasible to examine all reported costs and significant data reliability problems existed, GAO was not able to determine the extent that total costs were misstated.