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?