of the major reasons why early results have been uneven. District administration and teachers have, in many cases, lacked the appropriate professional development, instructional materials, and Common Core-aligned assessments to support effective implementation of the new standards. Adoption and implementation by states The chart below contains the adoption status of the Common Core State Standards as of March 21, 2019. Among the territories of the United States (not listed in the chart below), the U.S. Virgin Islands, Guam, the Northern Mariana Islands, and the American Samoa Islands have adopted the standards while Puerto Rico has not adopted the standards. As of May 12, 2015, five states have repealed Common Core. Nine additional member states have legislation in some stage of the process that would repeal Common Core participation. See also 21st century skills identified as being required for success in 21st century society and workplaces by educators, business leaders, academics, and governmental agencies No Child Left Behind, federal law requiring states to develop assessments in basic skills New Math, controversial attempt to revise mathematics education in post-war United States. Outcome-based education References Further reading Hess, Frederick M. and Michael Q. McShane eds. Common Core Meets Education Reform: What It All Means for Politics, Policy, and the Future of Schooling (Teachers College Press; 2013) 232 pages; Essays by academics and policy analysts on integrating Common Core Standards with existing efforts at accountability and other reforms. Pattison, Darcy. What is Common Core? (Mims House; 2013) 78 pages; Overview and introduction to the Common Core State Standards. Richard P. Phelps and R. James Milgram, The Revenge of K–12: How Common Core and the New SAT Lower College Standards in the U.S., Boston: Pioneer Institute, 2014. Tampio, Nicholas. Common Core: National Education Standards and the Threat to Democracy (Johns Hopkins University Press; 2018); Describes the history, philosophy, content, and controversy surrounding the Common Core standards for English language arts and math. Phelps, Richard P. Common Core Collaborators: Six Organizational Portraits Nonpartisan Education Review / Articles, 2018; Historical, financial and media analyses of the organization that spawned the Common Core Initiative, the two copyright holders, two of the paid proselytizers, and the delivery vehicle. Milgram, Stotsky, & Wiliam The Common Core Dissenters Nonpartisan Education Review, 2013; Includes explanations from three of the four members of the Validation Committee who refused to sign the committee report's recommendations. Nelson, Eric A. Cognitive Science and the Common Core