Oftentimes, students who have just joined the site have quite the learning curve ahead of them. When these students jump into grading immediately, they usually don't bother with figuring out how the grading system works right off. If only one week old or two week old students could grade, it would give them an opportunity to read the grading guidelines and get an accurate feel for what assignments deserve as a grade even through their own submitted work in that initial period. I have a feeling the number of grades marking down for detail, grammar, and unrequired information would go down quite a bit.
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