Say you are writing a letter of recommendation for a freshman student of yours who is applying for an undergraduate research position in a field very different from yours. You can of course say some things about how well she did in your class. However, you had a student that this person reminds you of who went on to do great undergraduate research in the exact specialized field that she is applying for, getting two first author publications. Would it be inappropriate to spend a paragraph comparing the two students and how that in your class they shared all of these great qualities that you think were the driving force for the previous student doing well as a researcher. This seems like it may be a bit unorthodox, is it advantageous or perhaps utterly inappropriate to compare students like this? I'm split, ideally I should just be focused on her but the fact that she reminds me of this student is really the most compelling argument I think I can make.