What fraction of new assistant professors end up receiving tenure at the institution that they started at? This of course varies by field and rank, so please include the area and range of ranks you are thinking of when you answer. (i.e. "Computer Science, top 25 but not top 10", etc.)
It seems hard to find hard statistics on this question. When you get official statistics for how many people come up for tenure and are denied, you get an overly rosy picture, because people who are not going to get tenure often leave before they come up for it.