sentences:  When failed subjects are reported in written, they normally have a score of NA or N/A, standing for "No Acreditada" (Not Accredited) or "No Aprobada" (Not Approved).Major works, such as a written thesis or doctoral dissertation, may not have a grade, but appear in certificates simply as AC, standing for "Acreditada" (Accredited).Occasionally, institutions, specially private schools, may use their own grading system, but there must exist conversion rules to convert those grades to their equivalent in the decimal system.