Referees are a system, not a formality

Chosen carelessly, they are the step that quietly fails.

Referees are usually assembled at the last moment from whoever is available, and the consequences are invisible to the applicant because a lukewarm reference produces silence rather than feedback. It is one of the few parts of the process where an applicant can be defeated without ever knowing it happened.

The controllable factors are straightforward. Ask before listing somebody, every time, including people who have agreed before. Tell them what the role is and which aspects of your work are relevant, so they can speak to the right thing. Choose people who observed your work directly over people with impressive titles who did not.

The most common error is listing somebody senior who barely knew you, on the theory that seniority carries weight. A vague reference from a director is weaker than a specific one from a supervisor, because specificity is the thing being sought.

It is also worth keeping the relationship warm before you need it. A referee contacted out of nowhere after four years gives a reference from memory, and memory has faded in ways that show.

There is one referee question worth asking that people avoid, which is whether they are able to give a strong reference. Phrased that way it gives the person a graceful exit, and somebody who hesitates has told you something valuable at no cost. The alternative is discovering it through an outcome you will never be given a reason for.