An article in Physics World has more information, and appears to be the source of the figure in question (I'm not entirely sure if it is the original source, as the article draws it data from elsewhere). The article is available to subscribers, and the full citation is:
- Harris, Margaret. "The academic pyramid." Physics World 25, no. 10 (2012): 54-57.
It appears to be presently mirrored here, and my information is drawn from the mirror. It answers some questions on data sources that were lost when the image got its own life without proper context. Data are for STEM fields, and are relevant for the United Kingdom. The figure caption reads:
Transition points in typical academic scientific careers following a PhD. Based on data from the Higher
Education Funding Council for England, the Research Base Funders Forum and the Higher Education
Statistics Agency’s annual “Destinations of leavers from higher education” survey.
Furthermore, the article states:
suggest that the vast majority of people who
complete science PhDs will never obtain a
permanent academic post. This is vividly
illustrated in a diagram published in 2010
by the Royal Society as part of a report on
the future of scientific careers in the UK
(figure 1). Drawing on data from various
UK sources, the diagram follows a “typical
academic career” through a series of post-
PhD transition points, when large numbers
of people leave the university environment
for careers in, say, government or industrial
research. These data show that less than
0.5% of science PhD students will ever
become full professors, while just 3.5% will
obtain lower-ranking permanent positions
as research staff at universities.
For physicists, that 3.5% figure is probably a little low. Slightly older data collected by the Institute of Physics and the
US National Science Foundation suggest
that the fraction of physics PhD students
who obtain permanent academic jobs has
historically hovered between 10 and 20%.
(...) But many more do want to stay in academia:
Indeed, according to an August 2012 survey carried out
by the American Institute of Physics (AIP),
nearly half (46%) of new physics PhD stu-
dents at US institutions want to work in a
university. The next most popular career
plan among those surveyed, attracting 18%
of responses, was “unsure”.
For more information, the article points to the UK group Vitae, UK science advocacy group Science is Vital, and the US NSF Statistics page.
So, for physics, it appears between ¼–½ of PhD students who want to get permanent academic positions, ultimately succeed in doing so. That's a quite different figure than 0.5% (but still problematic, as the article discusses in some detail).