In recent years, scores on the annual state reading and mathematics tests used for accountability have gone up in most states. These trends in state test scores do not always coincide, however, with trends on the National Assessment of Educational Progress (NAEP), the federally sponsored assessment that is administered periodically to representative samples of students for the nation as a whole and for each state. Consequently, questions arise about which set of assessments is more credible. This study by the Center on Education Policy (CEP), an independent nonprofit organization, analyzes whether state-level trends in NAEP reading and mathematics results contradict or confirm trends in state test scores. The study focuses on the 23 states with sufficient state test data, meaning states that had comparable data on percentages of students reaching proficiency for 2005 through 2009 for at least one grade/subject combination. For reasons explained later in this report, the authors compared trends between 2005 and 2009 at grades 4 and 8 in the percentage of students scoring at or above the proficient level on state tests and the percentage scoring at or above the basic level on NAEP. They also analyzed achievement on state tests and NAEP using an indicator based on mean, or average, test scores. They found more agreement between trends on state tests and NAEP than is commonly acknowledged. In general, the majority of states with sufficient data showed gains on both their state test and NAEP. The size of the gains tended to be larger on state tests than on NAEP, however. Here are the main findings from the study: (1) Since 2005, test scores have increased in most states with sufficient data; (2) Within the same state, trends on NAEP usually moved in the same direction as trends on state tests; and (3) Gains on state tests tended to be larger in size than gains on NAEP. Appendices include: (1) Tables with Specific State Names for Data Included in This Report; and (2) State-by-State Percentages Proficient/Basic on State Tests and NAEP. (Contains 11 tables, 1 box and 9 footnotes.)