'The Nation's Report Card' is a great measure for assessing how students perform on rigorous content. Historically, 'proficient' in state assessments was defined as 'at grade level' versus NAEP 'proficient', which means 'competency over challenging subject matter'. For more information about NAEP's achievement levels, click here.
NAEP was established by Congress in 1969 to measure educational progress in America and it is an indicator of what our nation's students know and can do in certain subject areas. NAEP asks the same questions and is administered in the same way in every state making it a reliable and valid measure for state-to-state comparisons. As a cautionary, NAEP, by design, never reports school-level or student-level results. Furthermore, NAEP cautions data users against interpreting NAEP results in a casual sense. There are many variables that influence students' academic performance (Source).
NAEP has publicly available items with state-level results to illustrate how states compare to one another and what students in Montana know and are able to do on these items. The reported differences in the graphics shown here may or may not be statistically significant, users are encouraged to go the Main NAEP Data Explorer to calculate significance.