So I will be applying to graduate schools in Computer Science this fall. I am an undergrad senior currently.

Last summer, I participated in an REU. I am wondering whether I should get my REU mentor's recommendation letter or not. The thing is that it was an REU outside of my college, so my mentor does not know me that well apart from seeing me in the 9 weeks we worked together. Am I better off just taking letters from people from my home college and those I have taken a good number of classes with?

I already have another summer research with a prof from my college so he will write one letter to talk about the research results. Can I exclude my REU advisor and take the other two letters from professors who taught me? What should I do? The REU was alright I guess; I feel like I did okayish during my stint.

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    You choose the references who know you best and think the best of you. Commented Aug 29, 2022 at 1:43
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    What is an REU?
    – shoover
    Commented Aug 29, 2022 at 2:40
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    “Research Experience for Undergraduates”: it’s an NSF-funded program for undergrads to do summer research with someone at another institution.
    – Danica
    Commented Aug 30, 2022 at 9:52

2 Answers 2


Short version: I wouldn’t consider it a red flag or anything, but if things went reasonably well then the REU letter is likely to be more helpful than a course-based letter.

I’m a computer science professor who’s been on the other side of admissions several times now. When I read rec letters — and I don’t think this is unusual — the biggest signal I’m looking for is evidence of being motivated for / good at your research experiences so far. This is usually hard to glean from anywhere else for undergrad applicants: there often isn’t a publicly available document of the research like a paper, or if there is one it’s very unclear what the undergrad actually contributed to this multi-author project and writing. A research letter can ideally say what working with you was like, which is likely to have a lot of bearing on what your potential advisor working with you would be like. If your REU was like mine, then you might have had a lot of contact and talking about ideas with your supervisor, and even in a short period of time this can teach them a lot.

Doing well in classes is of course a positive signal, but letters that say “student X was in my classes CS 201, 301, and 333 and got all As, very impressive!” add essentially nothing over just a transcript. If it’s someone who you’ve talked to a lot, especially about going deeper beyond the course material or having done a cool class project or anything like that, it can be a positive sign, but it’s hard to weight as strongly as a research letter imo.

Another factor that may be relevant is if your REU supervisor is a more established researcher in the field than the professor you’re weighing against. “This kid is really smart” letters from, say, a professor at a small liberal arts college (if that’s the kind of place you attend) who doesn’t work with many grad students are harder to calibrate, at least for me (incidentally an alum of a small liberal arts college), than someone whose publication record I can look up and get a quick idea of the kinds of students they’re likely to be familiar with.


Your application package should cover more than your coursework experience and academic success, which is something an admissions committee can read right off of your transcript. They don't need an instructor to tell them you got an A.

Your promise as a researcher will be something an admissions committee will be very interested in. There may be other ways you can cover this elsewhere in your applications package, but I recommend that at least one of your recommenders be able to speak to this in a letter, whether it is one of your local profs or your REU mentor -- though if you has a positive interaction, I'd really encourage you to ask the REU mentor for the letter, unless your other potential letter writers are stellar. The REU program is quite a feather in your cap, and you should highlight it.

If you choose to go with your local profs, I would recommend not going with someone who's entire interaction with you was having you as a student in a course, unless there's something more than you just sitting in a room and getting an A on homeworks and exams -- again, your transcript will cover that. You should focus on profs you've interacted with extensively during a course -- especially for those courses you've done an extensive project in, and the prof watched your progress on the project. It makes for a more meaningful letter.

You didn't talk too much about why you don't want to ask the REU mentor, other than the limited 9-week interaction. A 9-week direct interaction is a LONG interaction, but if your mentor runs a large group, and you didn't work directly with them, it is possible the interaction was, indeed, limited. If this is what you're worried about, there may be a person in the formal mentor's lab that might be a more appropriate letter writer than the mentor themselves -- maybe a high level grad student or postdoc who you worked with directly.

If your concern is that you don't want to bother your mentor, just put that right in the trash!! Mentors in these programs have an expectation that they'll be solicited for letters. Also, one shouldn't develop a reticence to ask for support when you need support.

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    In CS at least, I would not ask a grad student to write a recommendation letter, and probably not a postdoc. OP could certainly suggest to the professor that it might be helpful to talk with a particular student/postdoc who interacted more closely with them, though.
    – Danica
    Commented Sep 1, 2022 at 22:08
  • Sorry, it is a bit late to reply. My REU mentor was quite passive-aggressive at times. He doesn't reply to emails a lot. Even when I ask him for help on editing and making sure the abstract that I had to submit to apply for a conference is fine, he didn't reply even though I sent him 2 emails. When he does, his replies are cold. I am worried that even though the work was decent and I have it on my SoP, not having my mentor write a letter will hurt. What do you think about it? I have written about the work on my SoP and resume, but I might not have a mentor talk about it, which I am worried abt Commented Oct 15, 2022 at 3:07

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