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While we use rubrics to judge how well how well learners have met appropriate and clear expectations set for them and to give them feedback about their performance along a quality continuum, we also know that generally we are expected to issue grades as a descriptor of student performance.
Teachers often construct their own scales for converting rubrics to grades. As with rubrics, students need to be clearly informed of what the lowest "acceptable" score would be. The Fairfax County (VA) Public Schools World Language web site provides one model of a formula for converting scores from a point-based analytical rubric to a “grade.” Choose Performance Assessments for Language Students (PALS). Under each level, you will see a conversion chart for each rubric and task type listed.
The assessment section of the Nebraska Foreign Language Frameworks, available in PDF format, also features an assessment score conversion chart for converting a series of raw scores to percentage scores. The chart is found on the last page of the pdf document.