Nearly three out of 10 full-time PhD students have still not completed their doctorates seven years after starting their studies, research revealed today.
And figures for part-time students are much worse, with only one third submitting a thesis, making a part-time doctorate a "high-risk venture", according to the Higher Education Funding Council (Hefce).
Completion rates vary considerably between universities, noted the study, although the research did not identify the best (or worst) places to study.
The research also identified big differences between subjects - students doing natural sciences and maths had much better completion rates than humanities and social sciences.
Agriculture came top of the doctorates league, with 76% of students completing the course, compared to an overall average of 62%.
The study, which traced the progress of more than 19,000 students between 1995-96 and 2002-03, found that students were more likely to gain a doctorate if they had financial backing, particularly from research councils, charities or the British Academy.
Overseas students did better than average and so did younger students, the study found. So, unsurprisingly, did students who gained a first in their undergraduate degrees. Men and women had similar success rates, in contrast to school and first degree performance where males tend to lag behind.
The study grouped subjects into three areas - natural sciences, humanities and social sciences and vocational subjects including computing, engineering and business. It pointed out that the natural sciences, including agriculture, have well-established research fields with largely agreed methodologies.
"In these subjects, identifying topics and questions for PhD students is usually relatively straightforward. Fields of research in social sciences and humanities are not always as well established as in the natural sciences, and methodologies may still be disputed," the study said.
"Sometimes it may be difficult to identify topics which can yield substantial results through a PhD research programme. Completion rates for students on programmes in these subjects are not universally low, but when we take account of other factors we see that the 'subject effect' is negative compared to the natural sciences and related subjects," it added.
In vocational subjects, where the completion rates varied widely, the report mentioned alternative careers to research within the same field, for which a student with a partly completed PhD could be well-placed to enter. Computing, for instance, has a 50% completion rate for full-timers and only 23% for part-time students.
"Should these figures be cause for congratulation or concern?" pondered Hefce. "In part this depends on whether an uncompleted PhD programme is judged to be of value. This will vary from individual to individual. What is clear is that starting a part-time PhD is a high-risk venture: we can estimate that only one in three students is likely to submit a thesis within six years."
The report said that making meaningful international comparisons from currently available sources was not possible, and that it was difficult to know whether UK universities were doing better or worse than those in other countries.