If "vocational" is sometimes seen as code for "second rate" in higher education, then much the same thing can be said for "professional".
Although professional doctorates do not pretend to be PhDs, and very few students who pursue them would be interested if they did, it is perhaps unsurprising that the qualification is often compared unfavourably with its close relation.
"I think snobbery is an old English disease: in this case, the idea that theory is higher status than practice, and that research is more important than teaching," says William West, who leads a professional doctorate programme in counselling at the University of Manchester.
He is frustrated because, for their advocates, professional doctorates meet different needs from PhDs.
While the latter are widely accepted as the gold standard for young researchers pursuing careers in academia, professional doctorates, which are also known as taught doctorates, aim to open up university-level research to a group that might otherwise be excluded: career men and women who want to contribute not to the wealth of knowledge in an academic field but to professional practice in their area of expertise.
As John Taylor, professor of higher education management and policy at the University of Southampton, explains: "Professional doctorates are a way of extending the range of opportunities for people to do research; they are an important way of getting people in work to undertake research in universities."
This distinction is crucial when evaluating the qualification, as it ties in closely with the current vogue for employer engagement in higher education, a government priority driven by the recommendations of the Leitch review of skills in 2006.
For Bob Burgess, vice-chancellor of the University of Leicester and founder of the UK Council for Graduate Education (UKCGE), the ability of the professional doctorate to dovetail between research and the professions puts it in the ascendancy, particularly in a time of economic crisis.
"One of the characteristics of the professional doctorate is that it is designed to meet not only academic but also professional and vocational purposes. It seems to me that they fit the spirit of the age even more now than they did in the early 1990s," he says.
"In terms of employer engagement, clearly professional doctorates have a lot of potential because students take taught courses that are relevant to the area in which they are working, and their project or dissertation is usually practice-based and so speaks to the world of work.
"You would have thought that in an economic downturn, many professionals would be thinking about how to improve their skills, and the professional doctorate is an excellent way to do that."
The significance of this point is not lost on West either, who laughs wearily when asked to respond to those who dismiss the professional doctorate.
"There is a value in philosophical thought, but there's a point when it has to come back to real life," he observes.
But he is also cautious about marching too rigidly to a political tune, and insists that the professional doctorate is built on firmer foundations than fad or fashion.
He says: "I wouldn't want to chase government approval or grant money. We need to be independent and research the areas that we feel are important, regardless of whether or not they fit with Whitehall's priorities, but I also think that it's important that the work we do is useful to people at some level.
"I've learnt that there are fashions in education, and what was popular 15 years ago might suddenly come back in, so you can't rely on the latest government or research council line - you have to think longer term than that."
Burgess adds: "In the 21st century, higher education needs to think about how it is going to speak to a changing world and its requirements.
"The professional doctorate opens up opportunities for higher education to talk with professional people who are interested in intellectual problems that arise from their work experience, and that seems to me to be appropriate."
In spite of this optimism, it is apparent that the professional doctorate is not always seen as a welcome addition to a university's offerings.
For example, when the University of Cambridge proposed introducing a doctorate in engineering, the EngD, in 2005, there was opposition to the plan in its senate.
A researcher whose comments are recorded in the Cambridge University Reporter, the institution's official journal, described the decision as a "watershed".
"First they came for the polytechnics, to turn them into universities; but we were not a polytechnic, so did not speak up. Then they came for all the universities, to make them fulfil the function that the former polytechnics had, and we found that we were very sorry we hadn't spoken up," she said.
"Are we to accept that some of our schools, faculties and departments are glorified technical colleges? I have heard people expressing the opinion that they should be; it could, on the other hand, be considered to be submitting to the will of the Government were we to provide these vocational doctorates, which are very much a step down that path.
"Is all of modern academia to roll over and accede to the whims of those currently in power? The founders of this ... 800-year-old university would no doubt be appalled were we to do so."
Despite this impassioned opposition, the EngD was subsequently adopted not only by Cambridge, but also by the University of Oxford and most of the other Russell Group universities, too. It is now widely regarded as one of the most academically prestigious examples of the qualification.
Gerry McKenna was an early supporter of the professional doctorate, championing the qualification as vice-chancellor of the University of Ulster in the 1990s.
He maintains that they are an important alternative to the PhD, but believes that it may take time for those who dismiss them to see their value.
"We must remember that in the 1950s, particularly in the more academically strong universities, the PhD itself was looked down upon, so taught doctorates have to fight in terms of credibility," he says.
Despite remaining an advocate, McKenna is worried that the unstructured way in which professional doctorates have proliferated over the past 15 years has weakened their claim to parity of esteem with the PhD.
He says: "Taught doctoral programmes continue to languish behind the PhD in terms of credibility and status. One reason for this credibility gap is, unquestionably, their newness. It took the PhD many years to become accepted as a high academic achievement and the standard entry qualification for admittance to academic careers.
"A further factor has been the lack of clarity over what a taught doctorate is. Irrespective of discipline, there is international recognition that PhD holders have completed a substantial piece of original research, which makes a significant contribution to knowledge.
"The taught-doctorate holder cannot claim this, and the argument that the qualification involves doctoral-level scholarship and learning is diminished by the disparate regulations, frameworks and assessments that govern such qualifications between and within universities."
McKenna suggests that the professional doctorate has been "taken over by the teaching community", with not enough input from researchers.
He believes that the proliferation of titles and nomenclature has further muddied the waters, pointing out that while there are only two titles for Doctor of Philosophy - PhD and DPhil - there is a plethora of abbreviations for professional doctorates.
The result, he fears, is that while employers know, or think they know, what they are getting with a PhD, the same cannot be said for professional doctorates.
Denise McAlister, pro vice-chancellor for teaching and learning at Ulster, agrees that a lack of standardisation is damaging the qualification's reputation.
"We need to have descriptors that are recognisable nationally and internationally," she says. "At the moment, you can broadly say what a taught doctorate is: it's characterised by taught components; delivered to cohorts of students, unlike the traditional PhD; the field of study is professional disciplines; and students are generally supervised in a professional context.
"At that level there is a consensus about what they are, but if you look at the practice, there's no generic title such as PhD that captures the essence of the award. They are becoming more specific in title as the professions themselves become more specialised, so this (problem) is going to become more pronounced. It makes it difficult for employers to understand what they are getting."
But Burgess questions McKenna's view that everyone knows what a bit of paper with "PhD" written on it means.
"Is it obvious what you are getting with a PhD?" he asks. "As an employer, you would need to know the title of the thesis, for example, which is the obvious question to ask. The same question applies to a professional doctorate. You can't say that you know what you're getting with any doctoral qualification because there's a huge range of subject material and intellectual problems across both programmes."
West agrees: "PhDs are very different things in different subjects and at different institutions: how do you compare a physics PhD with a PhD in philosophy or art? They are all called PhDs, but they are very different. The obvious questions that are asked are: 'Who did you do your PhD with?' and 'What was the topic?'.
"These are important questions and allow people to distinguish between PhDs - the same is true with professional doctorates. The piece of paper tells the world that you have been examined to that level, but the actual meaning that someone might attach to your doctorate in the field will be based on these other questions."
One of the advantages of professional doctorates from the university's point of view is that they attract a different demographic than the traditional PhD does.
This can help universities to increase research student numbers, in turn improving their overall research profile. For the first time, students on professional doctorate programmes were admissible to the most recent research assessment exercise.
While McKenna worries that there are still people taking PhDs "who are not suited to them and who won't really benefit", at least there is now an alternative doctoral route, based in professional practice, that was not widely available 20 years ago.
Another potential advantage of the professional doctorate is financial, although commentators are divided on this point.
Southampton's Taylor says they can be a "nice little earner" for universities, but Graham Mills, who leads a professional doctorate programme in health and social care at the University of Portsmouth, disagrees.
"I don't think they are money-spinners, because they can be quite labour-intensive," he says. "However, a lot of these students are high-level people in their professions, so there are intangible benefits. For example, they might be able to offer placements to undergraduates, and it's seen as a good thing for a university to have such links with industry, even if the programmes themselves don't make much money."
As has already been indicated, professional doctoral students are a breed apart from their peers taking PhDs. The latter are usually younger, typically moving straight from an undergraduate degree or masters course to a PhD in their early to mid-twenties. But the professional doctorate's more mature demographic has specific needs and demands that can cause problems.
Professional doctorates are typically taken part time while students continue in their jobs. As a consequence they can take up to six years to complete, so dropping out is common.
Taylor, who highlighted this problem in a research paper on taught doctorates published in the journal Higher Education in Europe, says: "There are particular responsibilities on universities in the way they help and support these students, but I'm not sure in all cases that they have responded in the right way.
"I know from personal experience that these are wonderful students to supervise, because they're often experienced practitioners in their field and have a lot to offer. But they do not fit neatly into a pre-planned package: you often need to see them at the weekends, in the evenings and during holidays.
"You can't just assume the student is going to come to see you - you may have to go to see them in their workplace. That degree of flexibility and sensitivity isn't always apparent."
His argument is backed up by Manchester's West, who says "a lot can happen in six years, and it usually does", while David Rogers, the former dean of science at Portsmouth, describes students who take professional doctorates as "people in a hurry".
"I don't mean that they want to rush through their doctorates necessarily, but they are people on the move in their careers," he says.
"Their skills are probably better than the average PhD student that I have supervised, and they bring more reflection to the table. They are different animals - a PhD student doesn't complete and then immediately run a big research group, but professional doctorate students are probably already in senior positions in their field."
Taylor worries that non-completion will become more of a problem if universities start to expand their programmes beyond local provision.
"It is going to be interesting for universities if they start to deliver professional doctorates online or through other remote mechanisms. The challenges could be magnified even more in an international setting. It is far more difficult to be sensitive and flexible with a student who is 10,000 miles away."
Whatever lies in store for the professional doctorate, it is worth noting that, although its present-day popularity is relatively recent, its history is not as brief as one might assume.
Mills, who has run professional doctorate programmes for a decade, says that the qualification actually predates the PhD.
"Doctorates in law and medicine existed in medieval times in Oxford and Cambridge, whereas the PhD dates only from the turn of the last century.
"So the PhD is not a particularly old qualification, but at some point it became a one-size-fits-all option - it was the only thing available. It wasn't until the mid-1990s that people started to realise that in some cases the PhD wasn't fit for purpose.
"Industry found that doctoral graduates did not have the skills they needed, and that led to the PhD being re-examined. It was then that the professional doctorate, which had been around in the US and Australia for a while, started to take off again."
Its historical roots aside, it is certainly the case that the expansion in professional doctorates over the past 15 years has been rapid.
Figures from the UKCGE suggest that the number of professional doctorate courses in the UK doubled to about 200 between 1998 and 2005, while student numbers topped 6,000. The qualifications are now offered by a majority of UK universities. The most recent UKCGE survey, which dates to 2005, shows that of 83 institutions polled, 59 offered professional doctorates. Some 28 of the 34 pre-1992 institutions surveyed offer the qualification, and 31 of the 49 new universities that responded.
There is no reason to believe that expansion has ceased, but it is apparent from the concerns voiced by the qualification's supporters, never mind its detractors, that action needs to be taken to safeguard its future.
Taylor says: "The PhD is, quite rightly, still the gold standard, the thing that universities cherish most in maintaining standards of research, but there's confusion over the PhD and the professional doctorate.
"My view is that there should be certain expectations of a doctorate, such as the production of original research and development of research skills. You can get that through a PhD or a professional doctorate. My problem is that I do not think all professional doctorates meet these criteria.
"If you look at what the Quality Assurance Agency (QAA) has said, it does not distinguish between a PhD and a professional doctorate, it just sets out expectations for a 'doctorate'. I think that is a perfectly acceptable way forward.
"Where I have a problem is that I don't think this is pursued by all universities in all professional doctorate programmes. To me, a doctorate has to be about creating original knowledge, and I'm not sure all programmes meet that criterion.
"Such devaluing causes all professional doctorates to be called into question, which I don't think is fair. I can think of professional doctorates that are very strong. They are research-based with a high research content, but they serve the needs of employers.
"Unfortunately, that is not always the case, and that is a chink in the armour. It devalues the brand, if you like. In the end, universities have to police these things. The QAA has set out guidelines, but self-regulation has to be tougher."
If comparing PhDs and professional doctorates is inevitable, there is also a sense that, despite their commonalities, it is a bit like comparing apples with pears.
Manchester's West explains that the PhD is more open-ended and says it is important that people continue to take a research topic and run with it - that, perhaps, is how truly extraordinary breakthroughs are made.
But there is also a place for a more focused doctorate, catering to professionals who already have a sharp sense of what they want to do in their field.
"The bottom line is that it is important that people are studying at this level, that research is being done. Maybe that is more important than what you call it," he says.
DOCTOR WHAT? AUSTRALIA'S UNCERTAINTY ABOUT THE PHD'S FUTURE
Australia's Group of Eight universities are debating the future of the doctorate and the lure of the American model amid fears of falling standards and poor literacy levels. Caron Dann reports.
While few in Australian academia would concede that there is a crisis in the quality of its PhDs, most would agree that the nature of higher-research degrees are under scrutiny.
The value of new types of doctorate has been heatedly discussed in closed-door meetings.
Purists want to stay true to the traditional British model, where candidates conduct research over a number of years and submit a thesis of up to 100,000 words.
Meanwhile, proponents of the American model, which advocates coursework alongside research, say this has far more relevance to intellectual discourse in the digital age.
In some disciplines, the PhD is being completed by published papers rather than theses, and there is also a push for universities to offer professional doctorates, particularly for business students.
The qualification is comparatively new in Australia. The country's first PhD was awarded by the University of Melbourne in haematology in 1948.
A PhD Quality Forum conducted by the Group of Eight, comprising eight of Australia's leading universities, met last year to "identify areas of potential collaboration to improve the quality of PhD training".
The forum, held at the University of New South Wales in Sydney, will now put together an operational work plan for PhD offerings in 2013.
Roger Read, associate dean (research and international) at the university's faculty of science, says that contrary to what some, mostly older, Australian academics claim, he believes the standard of PhDs is not slipping.
"In my experience, they are still pretty good. They are also more in depth and multidisciplinary in their approach. There's more collaboration across faculties and globally," he says.
He adds that New South Wales' science faculty would conduct a major review of its research quality at the international level next year.
Science PhDs at the university are increasingly likely to comprise published papers rather than a single dissertation.
Read says this is advantageous, because it ensures that work is available "in the public forum" and because universities are under constant pressure to boost their publishing profile.
He adds that science PhDs today focus more on research data and its analysis than on trying to place research into a single line of argument, as conventional theses require.
He says there is still "a lot of talk" about the inclusion of more coursework in PhDs and that this could include presentation and writing techniques. He admits that students' lack of English-language skills is an ongoing problem.
Gil-Soo Han, associate professor at Monash University's School of English, Communications and Performance Studies, agrees that low literacy standards are an increasing problem among PhD students in Australian universities.
"The level of literacy is disappointing. I've worked with PhD students whose literacy skills are no better than those of first-year undergraduates," he says.
Han believes that doctoral programmes should be flexible and that universities should consider American-style PhDs with coursework components, PhDs by published papers and professional doctorates.
"Different styles of doctorate suit different types of student. Traditional Australian and British systems have a sink-or-swim approach, particularly in the humanities and social sciences. Students are often left pretty much alone, as if they were already serious scholars in their own right.
"That's very different to science PhDs, where there is a clear kind of master-disciple relationship."
Han adds that Australian universities need to "catch up" with the international community when it comes to new thinking about doctoral studies.
"Australia is still lagging behind in terms of developing professional doctorates and coursework PhDs. There is still a tendency here to look down on that sort of degree.
"Australians sometimes see American degrees as inferior. That's the wrong way to look at them."