So I interviewed a few months ago at Duke and U of Michigan, and I decided against sending thank you letters because at the time I felt that it gave the wrong impression. As time has gone on, I have begun to regret not sending thank you letters to the interviewers. Do you think that this will seriously hurt my chances? Also, at Harvard I sent a single letter to the Dean of Admissions, rather than to individual interviewers. Was this a bad idea?