Debate is well under way regarding the efficacy of the "No Child Left Behind (NCLB) Act," including whether this bundle of federal rules and resources is prompting gains in student achievement. Spirited conversation will intensify as the Congress discusses how to adjust and reauthorize this ambitious set of school reforms. Both state and federal gauges of student achievement will inform this debate. This paper first asks whether state testing systems provide an accurate and consistent indication of the share of fourth-grade students who are deemed "proficient" in reading and mathematics. It compares the states' own estimates against federal determinations, based on results from the National Assessment of Educational Progress (NAEP). It reports for the first time, on comparable state testing results and trends since 1992. The report details, looking across 12 diverse states, a small improvement in the percentage of children achieving proficiency in reading, based on NAEP results between 1992 and 2005. But states estimated much higher shares of students reaching proficiency, compared with the NAEP results. It then details how children made greater progress in math proficiency over this 13-year period. Yet again, it discovered that state test results exaggerate the annual rate of improvement, compared with the federal NAEP results. This historical disparity between state and federal estimates of proficiency levels has not previously been illuminated over this range of states. But the phenomenon is not new. The gap does not stem simply from NCLB's unintended incentive for states to set low cutoffs for defining which students are deemed proficient. Instead, it shows that states have long claimed that a much higher share of students are proficient relative to NAEP results, even before NCLB created the incentive for states to set a low bar. Appended are: (1) Fourth-Grade Test Score Patterns in 12 States; (2) Sources for Test Data and State Accountability Policies; and (3) State Policy Milestones. (Contains 6 endnotes, 15 figures, and 2 tables.)