Are Update Letters Necessary?

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It depends on what the update's about. "I've continued doing the activities I already reported on my application" isn't a noteworthy update, "I've been selected for a highly competitive research internship" is. A particularly noteworthy update can help you out, especially if you've been waitlisted post-II. As for which schools accept them, e-mail the admissions offices of the schools you applied to and ask them directly.
 
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