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You can access assignment submissions that you set for anonymous grading in the Grade Center or on the Needs Grading
Steps
From the Grade Center - After the assignment due date has passed or all attempts have been submitted, access the assignment's column and click Grade Attempts. For columns where you enabled anonymous grading, all cells are grayed out so you don;t know who made submissions.
From the Needs Grading page - Filter the items that need grading to show only the assignment you want to grade.
In the User Attempt column, all identifying information is replaced with Anonymous Student and an attempt ID.
From an assignment's contextual menu, click Grade All Users to begin grading
Both access options take you to the Grade Assignment page where you view submission and grade inline as your normally would.
Sources and Relevant Links
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