Recommendation letters choices : is diversity an advantage? I am currently in the process of asking people for recommendation letters for a Ph.D. program application. I need specifically 3 letters. I am lucky enough to have multiple people that I think could provide me good recommendations. My dilemma is therefore how to choose between these persons. One thing that complicates my choice is the fact that I am switching from one scientific field to another, let's say from physics to neuroscience. I also did my undergrad at university A and my masters at B.

I know that I will almost surely send recommendation letters from :


Masters supervisor in physics at B
Previous internship supervisor in neuroscience now a professor at C


Now for the third letter, I have to choose from :


Physics professor for whom I was a TA at B
Physics professor that I was on a committee with at A
Philosophy professor at A (he would probably write a very good letter)
Previous employer in a relevant industry (work with them for a year between masters and Ph.D.)


I am asking about advice for my particular choice, but also more generally: for this kind of application, is it better to prioritize recommendation letters from different universities, different sorts of work within academia (teaching, administrative, etc.), different fields or to consider people outside academia?
I am currently applying for a PhD and had the same dilemma as you, so maybe I can tell you what reasoning I followed as it might help you as well.


What field are you applying in? If it is a very academic field with few industrial applications, surely recommendations letters from professors or research supervisors rather than previous employers are more adequate.
You should choose recommenders that know different parts of your knowledge or skillset - e.g. if they all say how good you are at one specific software or problem type it will just be repetitive.
In my opinion, recommenders that have worked with you more recently come across more convincingly than older professors or supervisors, as your skill and your interests have developed since they last worked with you.
If you are changing fields, a recommender from that same new field who thinks you are capable of achieving success in the PhD is a big advantage (your 2nd recommender in this case).


If I were you, I would probably go for the "Previous employer in a relevant industry (work with them for a year between masters and Ph.D.)" or "Physics professor for whom I was a TA at B", depending on how academia- or industry-focused you want your application to be. All in all, it has to suit your profile and statement of purpose.

Hope my answer was helpful. Good luck!