About 8% of the time. Definitely not more than 25%. For the brief details, read on.
While compiling data on who was hired for N. American tenure-track asst. professor positions in ecology and allied fields advertised in 2016-17, I noted when a search failed, or apparently failed. Sometimes the chair of the department told me the search failed. Sometimes I inferred it failed because the university put out a press release this fall listing all their new faculty, that didn’t list anyone who could’ve filled the position. Sometimes I inferred it failed because the university is advertising for the same position again this year. As of this writing, out of 235 searches (some of which weren’t filled by ecologists), 19 (8%) failed by these criteria.
Some of those inferences could be wrong. Maybe some new hires haven’t been announced yet because the searches are ongoing, or the new hires delayed their start dates. Conversely, it’s possible I missed some failed searches. There are another 54 positions I checked, for which I couldn’t determine the outcome. There’s no one currently listed on the department website who could possibly be the new hire, and there’s no other reliable information to indicate the search outcome. So the absolute upper bound on the frequency of failed searches in this dataset is 25% (73/289), but I suspect the true number is closer to 8%
I only know why a few of these searches failed. In general, searches fail for various reasons. Anecdotally, the most common is that the position was offered but the offer was declined, often after some period of negotiation. At which point it may be too late to offer the position to someone else, for instance because the other candidates already have accepted other offers. I know of one recent search at an R1 that failed because four different candidates turned down the job.* Other reasons for searches to fail include searches that are cancelled for budgetary reasons, and searches for which no offer is made because none of the candidates were deemed acceptable.
*This does not indicate that there’s anything bad about that job or that department. People decline offers for lots of reasons, the most common being another offer.
UPDATE: out of curiosity, do these results surprise you?
Some other recent posts with data on the N. American tenure-track faculty job market in ecology and allied fields: