Letters that inform a committee on whether or not someone should get tenure are really important because the deciders include people that do not know the field. They can be deans and department chairs from other departments and ultimately a university wide committee. Think of these people when you write a tenure letter. Explain things you might not explain to your closest colleagues.
In some respects this is a letter like any other, where you should make clear some standard things. As I mentioned in the cited post, you should give an overview, how you know the person, what they have discovered and why it is important, then make them come alive with a story or two and finally mention any special skills or techniques they use or have developed. But a tenure letter is special. It is a potentially 30 or 40 year commitment to someone’s entire active career. How can you make a judgement on that?
Elsewhere, I have written about the importance of helping someone get tenure. I view tenure failure as a social failure and an extremely expensive one, though it is perhaps not as expensive as a tenure mistake. But mentoring is done and you are now helping a department and university decide what to do next.
If you are an outside letter writer you should focus on what you know, the research career. Don’t try to interpret the teaching or service statements. Leave that to the home institution. Instead try to communicate the importance of the work. Most of the time I view tenure letters as advocacy pieces where you are explaining to a possibly unfriendly or distant administration why your whole field and this person’s work in particular is important. This can be fun and easy. You don’t have to read everything. Focus on 5 or 6 cool papers, taking care that some of them are very recent.
But you would not be doing your job properly if you did not alert the readers to possible warning signs. Has early productivity continued? Are there some cool recent ideas? Does the professor seem to be expanding or changing field or is he essentially repeatedly re-doing his dissertation? Growth is essential for continued productivity in this business. Look for new collaborations, new ideas, an active group. If the person’s fame is all from early on, tell the reader.
The first tenure letter I ever wrote I spent a week on. I read everything the person wrote. I pored over all the documents the committee sent me. To this day, I know that person’s early work well. I do not do this any more. You are not being fair to yourself and your own other commitments if you generally spend anything over 2 to 4 hours on this task. Remember, it is not micro details, but big picture that matters.
In fact it seems like the higher the recognition the shorter the recommendation. Some fancy national societies expect only a couple hundred words and a dozen or so references. You will write more than that and it may take some time to find the best dozen words, but don’t make the job bigger than it has to be.