Requests for letters of evaluation for tenure candidates started arriving during the summer. The process typically starts in the late spring or summer, when the dossier is assembled and the letters are requested, with letter due dates in the early fall so that departments can vote and pass their recommendations along up the administrative chain.
I almost always say yes to requests to write letters for tenure cases. In fact, I think I have only said no once, for someone who was too far outside my field and who had (in my opinion) been quite unproductive. I thought it best if people who were more expert in that field evaluated whether the quality of the papers more than made up for the low quantity of papers; I did not feel confident that I could do this.
Also, I was given a very short amount of time to write the letter, and could not have done a thorough job. I think it is important for these letters to contain something new and informative that cannot be gleaned from reading the CV, so writing these letters can take time.
Declining to write a letter for someone's tenure evaluation can be seen as a blot on their record unless you have a really really good reason for declining. Department and university P&T committees may pore over decline-to-provide-a-letter explanations, trying to determine if the declination reflects a negative opinion. It may not, but some suspicious faculty may still wonder: OK, so this person is really busy preparing for their Senate confirmation hearing, but if they really thought the candidate was great, wouldn't they take the time to write the letter anyway?
So I try to write the letter if it seems at all reasonable to do so in terms of my expertise and/or time.
Although most letters are easy to write in terms of content, some letters involve a bit of agonizing over wording and tone. For me, the most difficult letters to write are:
1. letters that are not entirely positive. In these cases, there is always a question of whether to accept or decline writing the letter in the first place. I think it is important to be certain that a negative letter is also a fair letter. If my opinion is not positive, is there a good reason for this? Do I have the necessary expertise to evaluate an individual's record? Can I be objective, or am I unduly influenced by an irrelevant dislike of the individual or his/her research topics?
2. letters for institutions that are not peer institutions. It is common to be asked to comment on whether the candidate would receive tenure at our own institution. If the candidate is at a peer institution, this is not typically a difficult evaluation to make. For candidates at institutions that have less emphasis on research and/or that have less awesome facilities and/or a smaller graduate program (and rare or no postdocs), this evaluation is more difficult. I've written before about how it can be challenging to state that the individual in question would probably not receive tenure at my institution, but is a good candidate for tenure at Lesser University. By definition, that's a rather patronizing and obnoxious statement. I have seen P&T Committees agonize over such statements, even if the rest of the letter is positive and even if the letter writer is at a university that is famous for eating its young (faculty). These statements are not to be made lightly or without justification.
3. letters for candidates up for early tenure for reasons that are not obvious. Faculty up for early tenure are evaluated by the same criteria and standards as those up for tenure at the normal time; you can't give them a break for not having as much time, nor can you hold them to a higher standard. Some assistant professor superstars get early tenure because they have such outstanding records; it's easy to write glowing letters for them. But what do you do if you get a dossier for someone who looks like they are on track but could really use another year or two to demonstrate that better? What if you look at someone's CV etc. and you have no idea why they are up for early tenure? Unless you are totally off-base with your criteria and standards, perhaps there are some departmental politics at work and you don't know what these are. These are difficult letters to write because you might have been able to write a really positive letter in another year or two, but as it is, you can realistically only write a somewhat vague letter about how this person shows some promise. In these situations, I just write what I think anyway, and let the department sort out its own political situation however it wants.
The letters from senior faculty at other institutions are just one element of a tenure dossier. I think it is worth putting effort into writing the letters, I don't believe that any one letter will either sink or save a candidate. It's just another data point, so I might as well make it a useful and interesting data point, without either over- or under-estimating the value of my opinion.