Times Higher Education (THE) have started publishing some basic university statistics on their rankings page: number of students, student-staff ratio, international students and female-male ratio.
Already some observers have noted that the data does not always match that found in institutional and official sources. I have heard that the number of students given for several German universities is significantly lower than that found in other sources.
The Online Citizen in Singapore has found that the island's two leading tertiary institutions, National University of Singapore and Nanyang Technological University, have claimed 34% and 33% international students respectively on the THE site although in 2013 the Minister of Education had claimed that the proportion of international students in Singaporean universities was only 16 %.
There are several plausible and innocent explanations for this and similar discrepancies. It could be that part-time students, branch campuses, online students, permanent residents, research institutes, commuters living in Malaysia are counted in one set of figures but not the other.
But there is a serious and general problem with institutional data for university rankings. Even if everybody concerned is completely honest, there are many points at which ambiguous definitions, conflicting estimates, duplication or omission of data can undermine the accuracy of ranking indicators. In the case of Germany there might be some argument over whether doctoral candidates count as students or teaching and/or research staff
QS used to have a validation hierarchy starting with national statistics, followed by institutional data, data from websites, old data, third party data and smart averages in that order. If it is still applied rigorously this would be the best approach.
I understand that both QS and THE reserve the right to overrule institutional data although how severe they are I do not know. THE have a particularly difficult task since they allow universities to opt in or out as they please. Should THE be too strict about the data supplied a university might simply decide not to be ranked for a year.
On balance, it is probably good sense for ranking organisations to rely on publicly accessible data when they can and to minimise input from universities.