Will a professor write you a letter of recommendation? My answer: Writing student recommendations comes with the faculty job, and I usually write if asked. But since this academic territory is often so unfamiliar to grad and undergrad students, let me give some general guidelines and advice.

(For general advice on PhD applications see this post, and for other PhD advice see the right sidebar, including How to Get a PhD and Save the World)

What do letters do?

Let’s start by asking how PhD admissions committees use letters (at least the ones I’ve been on). In short, a PhD is designed to train professional researchers. But grades and test scores and transcripts give a very noisy signal of your talents for research.

Here’s what a PhD program would like to know: Are you creative? Hard working? Do you ask interesting questions? Do you understand how research is done? What are your actual technical skills, and how quickly will you progress to doing original research?

A letter is designed to communicate this information. Without it, narrowing down 1000 applicants to a few dozen admitted students (or fewer) is extraordinarily hard.

Faculty take these letters seriously, since we often write these letters to our colleagues at the same pool of schools and employers. Our reputations are at stake.

All this is why good letters start out saying exactly how well we know the student (for how long, what kinds of interactions) and then discusses their classroom performance, research aptitude, skills, and talents in detail. As much as possible, I try to place the student relative to past students I have recommended to PhD programs, and their success, to give a very precise picture. (I wish more letter writers did this.) Altogether, good letters are often 2–3 pages long, and take a long time to write.

Who should you ask?

For all these reasons, strong letters usually come from long and close relationships with professional researchers, especially university faculty.

Signs a faculty member can write you a long, detailed, strong letter:

• You were a research assistant for them for an extended period of time
• They supervised your undergraduate or master’s thesis
• You took more than one class with them, and developed a good relationship in office hours, discussing research ideas

Signs that a faculty member can write you a short and weak letter:

• Letters from non-researchers (like a senior manager in the private sector)
• Letters from faculty who only know you from one class (and not that well), even if you did get an A
• Letters from the head of your program, whom you did not work with closely

Weak letters will not help your admission much, and may even send a negative signal.

Another common mistake: A lot of students think that it’s better to get someone well-known to write a letter, even if they don’t know the student deeply. In my view, it’s almost always better to have someone non-famous write a detailed letter, as long as they know you well.

Keep a few other trade-offs in mind. Assuming a faculty member knows you well, it is often better if they have a track record of sending students to PhD programs (so they can rank you, or so that you benefit from their reputation). But these same faculty may also be writing multiple letters each year. And they might not know you as well. It’s a tricky balance. Just be aware of the trade-offs, and always ensure the faculty member at least knows you well.

Granted, students don’t always know three professors that well. (I didn’t, the first time I applied.) Therefore, often an applicant’s letter is a bit weak. That’s a familiar sight in applications, and it’s normal. But you definitely want a couple of detailed letters.

Other people have the opposite problem, and know several professors. If your application requires three letters, you might be tempted to send four. This is usually a bad idea: every additional letter after your best one lowers the average quality of your application. This is simple arithmetic. A fourth letter only makes sense if it will be glowing and adds a completely different perspective.

If you want to improve the quality of your letters, taking a Master’s degree before the PhD is an option, but I think it’s far better to work as a research assistant. You will probably learn as much or more, and you get paid rather than fork over cash to a university.

Last: How can you find out if someone will write a strong letter for you? One way is to ask. Personally, I am frank with students, so as not to do them a disservice. Honestly I think most professors will tell you in broad terms what kind of letter they can write.

What to do once a professor has agreed to write a letter

Good letters also take time to write, especially those for graduate school and (most of all) for the academic job market. Approach your professors as early as possible–ideally at least a month in advance. Faculty are extremely busy, and last-minute requests might get last-minute effort.

Once a professor has agreed to recommend you, three or four weeks before the due date you should email the following:

• your latest CV
• your transcripts, GPAs, and (if relevant) GRE scores
• a list of where are the letters will be going, and
• depending on where the letter is going, some supplementary information:

Grad school, scholarship, or academic job market applications: share your statement of purpose, research proposal, or other personal statements. Letter-writers take these seriously, and often discuss the specific strengths and weaknesses of the proposal in their letter. So try to ensure you letter writer sees a fairly polished version before the letter is written. This means you probably want to start discussing the proposal with your advisors at least six to eight weeks before the letters are due.

Job or fellowship applications: include a description of the job or fellowship, and what they are looking for in a letter. Often a job or scholarship application will offer specific guidelines. If they don’t, provide a quick description to your letter writers.

Try to send everything in one package, well organized. We letter writers are absent minded, and will probably lose the multiple pieces if you do not collate them for us.

Most grad school applications are online nowadays. This is less convenient than it sounds, since every school has a different custom form. Please fill out the maximum amount of information about your recommender that you can (including contact info) so that your recommender doesn’t have to complete the same info 13 times. This process takes a faculty member hours and hours.

For grad school applications, you want at least one to two academic letters. Two is probably better, with one from an employer if you have been out of school for long. So maintain contact with your advisors after you leave university.

Comments, criticisms and questions are welcome below.