A culture of secrecy and mistrust among private providers is severely hampering research into higher education, academics have claimed.
Speaking at the European Association for International Education's (EAIE) annual conference, Philip Altbach, director of the Center for International Higher Education at Boston College, said efforts to study private and for-profit universities had been hindered by a lack of institutional cooperation.
"To get data about what they are doing - what their motivations, costs and strategy are - is nearly impossible," said Professor Altbach at the conference in Copenhagen.
He said the lack of transparency had impeded a vital area of research.
There are around 250 journals devoted to research into higher education. Around half are based in China and about 60 in the US, with others published in the UK, Germany, Spain and Japan.
Stephen Wilkins, a PhD candidate at the University of Bath and winner of the EAIE's Tony Adams Award for Excellence in Research, said his work into students at branch campuses in the Middle East, the majority of which are private, had been defined by a lack of cooperation and even obstruction.
"Institutions do not want to cooperate, especially if you are doing cross-border research," he said.
"We approached five institutions to see if they wanted to help. Only one indicated an interest, but when we put in a request we waited six months before they eventually said no, too. I was forced to bypass the institutions and approach students directly. We used Facebook and online social networking to contact them and managed to get about 250 responses."
He added: "Even when I was trying to do a potentially positive piece on branch campuses, I approached several institutions and no one would help...If you say anything negative about the institution in your research, there is a good chance you are offending someone quite powerful."
Any attempt to research the movement of international students was viewed with suspicion, he said, and many institutions expected research to show them only in a positive light.
Hans de Wit, professor of internationalisation of higher education at the Amsterdam University of Applied Sciences and editor of the Journal of Studies in International Education, said the commercial premium on comparative data had made it difficult for researchers to access the information.
"There are lots of companies which collect data and you can only get this data if your institution subscribes and they charge very high fees. I'd like to see them open these databases for researchers."