A discussion on the different styles of PhD thesis that might be appropriate for different students and research areas.
I was talking to one of my students about the "shape" of their PhD, and how we might present the work they're doing in the final thesis. This particular student is working on the science of complex coupled networks, which is a very new area of network science -- itself a pretty new field. The exact details of her work aren't important: the important point is that it's an area that's so poorly understood that it's hard to ask meaningful detailed questions about it.
A PhD is a training in research: the work done in the course of one doesn't usually change the world, and a fact that's often forgotten is that they're not supposed to. The goal of a PhD is to demonstrate that the candidate can conceive of, plan, conduct, evaluate a communicate a programme of research, sustained over typically three or four years, that adds something to human knowledge. Clearly there are many ways one can demonstrate this, and the question we were asking was: what are the variations? How do we choose which is appropriate?
A typical thesis is a monograph, written specially to describe the research, its process and results. In this it's very different from a scientific paper, which typically focuses on a single result and often elides a lot of the motivation and process. Publications along the way always help, of course: they let a reader (or examiner) know that the work being presented has been reviewed by others in the field, which always gives a certain amount of confidence that it contains worthwhile results. (Few things make a PhD examiner's heart sink more than a thesis with no associated publications. It means you're on your own in terms of assessing every aspect of the work.) The monograph needs a "shape", a skeleton upon which to hang the research, as well as clearly-enumerated contributions to knowledge and an understanding of its own boundaries.
There are however a couple of different styles of skeletal structure.
The hypothesis. This is my favourite, truth be told. The work is encapsulated in a question, and the rest of the thesis seeks to answer this question. The answer might be yes, no, or maybe, all of which can be valid and can result in the award of a degree. The research then proves the hypothesis, disproves it, narrows it down as incomplete, or uncovers extra structure that would require substantially more work to explore -- or some combination of these.
There are several reasons to like this style of thesis. Firstly, the hypothesis is usually something concrete within the field of study. (It may seem hopelessly abstract to anyone outside the field, of course.) That makes it easy for an examiner or other reader to relate to: the question makes sense to them, as something one would want to know the answer to. Secondly -- and more importantly -- you have a pretty clear goal and consequently a pretty good notion of when you're finished. A PhD can almost always go on for ever, of course, but having a tight hypothesis means that you have a reference against which to test the work and decide whether it's sufficient to convince the reader of your conclusions.
Against this we may contrast the second type of thesis:
Adventures in your chosen field. In this kind of thesis, there is no hypothesis. Instead, the thesis marks-out an area of interest and explores it, with a view to understanding or characterising some or all of it but without any driving question that demands an answer. Unlike the hypothesis, this kind of thesis isn't really "for" anything: it's intended as an exploration, a search for features -- and indeed possibly for hypotheses to be tested in the future. But this style of thesis is perfect for some areas, like mathematics and the more abstract end of computer science, where something new needs to be explored for a while to see if it makes sense and what is has to offer. (It's also found in non-monographical theses, where some universities allow candidates to submit a portfolio of published papers in place of the traditional approach. I must say I always find theses like these quite disjointed and fundamentally dissatisfying: there's a lot to be said for a purpose-written, rounded monograph as a conclusion to a programme of research.)
One would probably have to admit that an "adventures" thesis is a little more challenging to examine, since it doesn't motivate its own intention as clearly as a "hypothetical" thesis. It's going to be harder for the candidate to decide when it's ready, too, since one can always do just one more search to find something else of interest. In that sense, an "adventurer" needs to be more disciplined both in deciding when enough is enough and in deciding what, of the many possible choices, constitute the interesting pathways to explore.
I don't think either style is inherently preferable. It depends entirely on the subject matter, and one could easily fall into the mistake of trying to construct a hypothesis for work that was really an "adventure", or indeed not constructing one for a thesis that really needed one to motivate and structure it. There's a clear difference between an exploratory thesis and a rambling one: the exploration needs structure and coherence.
It's also perhaps worth considering the psychologies and research styles of different students. Some students need concrete goals: to build something, to test something, to prove or disprove a conjecture. These people clearly benefit from doing "hypothetical" theses. Other students have an interest in an area rather than in any specific outcome, and so might prefer an "adventure". There's no point in forcing one into the other, any more than one can get a theoretically-inclined student motivated by coding (and the associated debugging): it's just not what floats their boat, and it's better that they know their own minds, strengths and interests than to spend three or four years engaged in a quest to which they're fundamentally misaligned.