The PRPC examination was developed by psychometrically-valid, legally-defensible processes. This includes the creation of a Job Task Analysis survey by a group of Subject Matter Experts, administering the survey to a large sample of practicing therapists in the field, psychometric analysis of the data to create a test blueprint based upon the knowledge and skills determined by the survey, item writing, beta testing of items and then administration of the exam. For this process, H&W worked with Kryterion, a psychometric certification development company, to ensure the highest standards of credentialing were being met.
It is a common misconception that board certifications created by the ABPTS (OCS, WCS, etc) are "approved" or "validated" by the American Physical Therapy Association. This is not the case. The specialization certifications developed by the ABPTS were developed by the same psychometrically-valid and legally-defensible methods for establishing the minimally-competent practitioner that Herman & Wallace used for the PRPC exam. The APTA is not a credentialing body; it does not validate, create nor approve certification exams.
There are a number of bodies which do accredit certifications, however, including The Institute for Credentialing Excellence (http://www.credentialingexcellence.org/). PRPC, and other exams created using the steps described above, would be eligible for accreditation through this body, based on the rigorous psychometric standards applied when creating the exam, as would other certifications created via this long and detailed method.
The validity of the board certifications offered by the ABPTS derives from the psychometric process by which the exams were created, as does the validity of PRPC.