A PhD Thesis, or PhD dissertation, as it is sometimes called, has a special function in the academic community. This written piece of text, typically amounting to 150-300 pages (Swales 2004, p. 102), functions as a kind of scholarly qualifying piece of work, through which the author is admitted into the society of academics seen as sharing some sort of common ground in terms of expert knowledge, skills, critical thinking, rigour, and scientific values.
Strictly speaking, there are two main types of PhD theses: the monograph format and the article-compilationformat. The monograph, which can be seen as the more traditional type, is written as a coherent, synoptic text, whereas the article compilation thesis (sometimes called multiple-manuscript thesis or anthology) features a series of published/publishable papers compiled into a coherent whole, often with designated introductory and concluding chapters (Swales 2004).
In terms of their spread and use, the article-compilation format is predominantly adopted in the hard sciences and engineering, but it does also occur in some disciplines in the social sciences and humanities. The monograph format is arguably the most commonly used in the soft sciences. On the whole, though, there is a lot of variation when it comes to the preferred format in different disciplines.
When it comes to the structure of a PhD thesis, there is naturally a great deal of variation going on across different disciplines, but one format is at least at the face of it expected to traditionally dominate: a blown-up version of the IMRD format often used in research articles. Click on the link below to see this traditional format.
However, as has been pointed out by Swales (2004), the above traditional structure is perhaps not the most widely used. He argues, for example, that one problem with the traditional structure is that it might be problematic to have only a single chapter dealing with results. This, he continues, "produces an unwieldy and out-of-balance monster chapter in the middle of the text" (p. 108).
An alternative structure for a PhD thesis is similar to the article-compilation format mentioned earlier. This format is seemingly more popular in science and engineering disciplines. Click on the link below to see an example of this thesis format.
A third structure, allegedly mostly used in the humanities and social sciences, but also found other disciplines (Swales 2004), is called the topic-based thesis. According to Bunton, in these types of theses authors "report and discuss their analyses in multiple chapters (ranging from three to seven) with topic-specific titles" (1998, p. 110). Click on the link below for a schematic overview of the topic-specific structure.