If someone is on a tenure track in a science or engineering department at U.S. institution (Tier 1, if that makes the question more focused), and it is time to review his case to be tenured and promoted to associate professor, what is a typical range for the number of the external reviewers for his file? Does the number vary if the case is for professorship promotion?
The actual number varies by institution and, frankly, does not matter that much. The real questions are:
- Are you known in a community? Do people in your academic community generally know of you and, more importantly, your work.
- Does the university want to keep you? It is always possible to find people who will write either effusively positive or cynically negative reviews of anyone and everyone. Such reviewers are generally known to people who seek their reviews.
The first question is loosely tied to the visibility of your research, and the second question is loosely tied to your value within the university (often including teaching, grant funding, collegiality, and the like).
At my institution, (which typically takes its cues from Tier 1 US institutions and is led by scientists who all come from Tier 1 US institutions), the dean solicits 6 names from the faculty member and another 6 names are determined by him or people in the program. From this list of 12, it's necessary to obtain at least 6 letters for the case. It goes without saying that none of the 12 names can have any significant ties to the candidate. All of the 12 should be full professors at "peer" institutions, which means institutions of the same or higher research caliber as your own. Typically most of them are highly distinguished, such as people who hold leadership positions in societies or journals or who have won major awards for their research.
The above is for promotion to associate professor. I believe that for promotion to full professor the number of letters is higher.
If you are interested in a particular institution, look up the rules in their faculty handbook (which is often publicly available).
The numbers vary enormously between universities and departments and, from my experience, seems to correspond at least roughly with the prestige (and tenure rate) of the institutions. At the very high end, I've met faculty in the humanities at both MIT and Columbia whose told me that their case required (or will soon require) 25 letters!
Many top large research schools including the large state university that currently employs me will request in the order of 4-8. I'm under the impression that the majority of universities will request between 2 and 5.