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How many references on average are typical for a PhD Thesis?
More specifically, I'm looking for a comparison of the average number of references for an engineering PhD thesis and PhD theses in other fields.
I'm looking for references or data on the subject.
As alluded to in the comments, there is no generic "right" answer: the number will depend strongly on the nature of your thesis, the state of the literature related to it, and your personal scholastic style. Nevertheless, in the spirit of engineering approximations, I will supply you with some rough guidelines.
As a lower bound, I would be startled if it was possible to place most theses within the context of pre-existing work in less than about 20 references. In certain contexts such as an obscure corner of pure mathematics perhaps this might be possible, but in most cases work does not take place in a vacuum, and other people will have either used similar techniques or cared about similar problems in the past.
As an upper bound, I would be concerned with a thesis with on the order of 200 or more references that the student hasn't been doing enough work on their own original research and has been investing too much time and energy in building a literature review, rather than a thesis. Again, there will likely be cases where a thesis with an extremely high number of references would be reasonable, but these are likely to be unusual.
Notice the extremely wide range and suspiciously round numbers in my estimates: really there is no "right" answer for how many there should be in general, but perhaps these will help with some initial first impressions in one's thinking.
A quick look at the mostly Mechanical Engineering theses I have on my computer at the moment gives an average of just over 100 references, with a range of approximately 80-150.
I haven't been able to find any quantifiable data but I feel this is about right. Some people will have significantly more and a few less.
I also suspect there is significant variations between subjects with arts and social sciences having significantly more references and more theoretical subjects having slightly less.
In essence, the number should be small enough to prove that the major composition of your thesis is your own work, yet large enough to satisfy the committee during your thesis defence.
As first stated, there is no actual upper or lower bound per se. But required number of items to be referred is more of a hidden factor that is highly dependant on your doctoral committee. Thus it would be best to consult your advisor and, if permitted so, members of your doctoral doctoral committee. In addition to that, you ought to look through some PhD dissertation accomplished by others in your field to get a good idea of the average number if that is what you really want.
It really depends on the subject. Imagine you are writing a thesis on three complex inter-related medical conditions and you include a well-sustained hypothesis for further study. It would not be very hard to reach 300 or more citations. You know that people reading your work will not be specialists on the three subjects at the same time, so you have to provide a good and comprehensive introduction on each subject, so that everyone can understand the results and discussions. This alone can take 200 or more references, some of them will be used in the discussion. Now imagine that two of these medical conditions have recent definitions and guidelines that are completely different from those from two years ago, and you have to contrast results with papers using the old guidelines, as well as some few papers that use the new guidelines. Now you have to explain both guidelines and in which way they differ from each other. Furthermore, the pathophysiologic models for the interaction between these conditions are complex and comprise systemic, organic, cellular, molecular levels, and you have to explain some of them in order to establish your hypothesis. This is where I'm headed... around 320 references and counting...
For me, I think that the use of references should not limit the creativity of the Doctorat candidate. Rather, they should be that springboard that get them produce more than what they get as intake. Numerically, I expect to have each single reference to be equated with two pages of the candidate's work as a maximum. This would mean that a dissertation of 300 pages should not have used more than150 references. The number wil' vary according g to the length of the dissertation. The higher the number of references, the more limited the originality of the work and the poorer the creativity displayed.
