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Problems with CREDO’s Charter School Research: Understanding the Issues

Reference Publication:
Review of Urban Charter School Study 2015

Andrew Maul's rejoinder to CREDO’s response


William J. Mathis, (802) 383-0058,
Andrew Maul, (805) 893-7770,

URL for this press release:

BOULDER, CO (September 28, 2015) – Earlier this summer, the Center for Research on Education Outcomes (CREDO) published a response to professor Andrew Maul’s review of CREDO’s Urban Charter School Study. The National Education Policy Center (NEPC) today released Maul’s reply, in which he thanks CREDO for the response yet explains, point-by-point, why he stands by the following eight concerns he had earlier raised about that study:

  1. The nature of the comparison between charter and traditional public schools in the CREDO studies is not clear;
  2. The matching variables used in CREDO’s studies may not be sufficient to support causal conclusions;
  3. Some lower-performing charter students are systematically excluded from the CREDO studies;
  4. CREDO’s reasons for the systematic exclusion of lower-scoring charter students do not address the potential for bias arising from the exclusion;
  5. The “days of learning” metric used in the CREDO studies is problematic;
  6. The CREDO studies fail to provide sufficient information about the criteria for the selection of urban regions included in the studies;
  7. The CREDO studies lack an appropriate correction for multiple significance tests; and
  8. The CREDO studies have trivial effect sizes.

Maul’s original review and his short rejoinder are published by the NEPC, housed at the University of Colorado Boulder School of Education. Maul, an assistant professor in the Graduate School of Education at the University of California, Santa Barbara, focuses his research on measurement theory, validity, and research design.

Find Maul’s original review of CREDO’s urban-charter report and his full rejoinder here.

The original CREDO report, and the CREDO response to Maul’s review, are currently available at the following urls:
Original report:
Peterson’s Response:


The mission of the National Education Policy Center is to produce and disseminate high-quality, peer-reviewed research to inform education policy discussions. We are guided by the belief that the democratic governance of public education is strengthened when policies are based on sound evidence.  For more information on the NEPC, please visit