When to refuse tenure and promotion external review request?

[mleok]

For me, I am inclined to say yes if I am in a position to offer a unique perspective on the candidate's research and how it relates to the broader field, and I respect the candidate's work. I wouldn't go so far as to write a review on a candidate whose research I consider to be mediocre, as even a single lukewarm letter may be sufficient to deny tenure. The candidate's research would have to be positively detrimental to the standards of the field for me to seriously consider tanking their tenure prospects single-handedly, and if there are members of my community who disagree with that assessment, then I would rather err on the side of caution and simply decline the invitation.

[mleok]

With regards to how many letters you should feel obligated to write over the course of your career, I believe in paying things forward. So, at the very least, you should write as many tenure or promotion letters as the number you received in support of your own tenure and promotion cases.