From an Inside Higher Education essay on the MLA committee on tenure's final report
(
http://insidehighered.com/news/2006/12/08/mla for the facts,
http://insidehighered.com/views/2006/12/08/berube for the committee member's essay):
MLA studies of Ph.D. placement show that no more than half, and often fewer, of any given year’s Ph.D.’s are hired to tenure-track positions in the year they receive their degrees. Information is sketchy for career paths beyond the first year, but what information is available suggests that, on average, something on the order of 60 to 65 percent of all English and foreign language Ph.D.’s are hired to tenure-track positions within five years of receiving their doctorates and an estimated 38 percent are considered for tenure at the institution where they were hired. Of those 38 percent, 90 percent - or 34 of every 100 doctoral recipients - are awarded tenure.
Feel free to pass this statistic on to anyone you know who is considering graduate school...
(For what it's worth, that stat more or less aligns with my experience at a top-ranked grad school. Of the 25ish people who entered my program with me, maybe half completed the degree, and three are in tenured or tenure-track positions. One is an adjunct. For the rest of us, our professions now include philanthropic fundraiser, political organizer, digital interface designer, and educational policy administrator.)