When you have got your results and you have presented them to the reader in your thesis you are left to answer the most difficult question of all: So what? The job of your PhD thesis conclusion is to show what contribution you have made to the field of knowledge.
Originality versus Contribution to Knowledge
Many people think PhDs have to be truly original. The problem with this aspiration is that most PhDs in practice build upon an existing body of knowledge. Most PhD students spend the first year or more studying it, and the early chapters of their theses reviewing it. This is not necessarily conducive to staggering originality. In most cases, PhD theses are designed to expand the current body of knowledge incrementally, rather than establish a whole new field of academic endeavour. This is best represented as a contribution to the body of knowledge. Think of a volcanic eruption as a metaphor. Most eruptions will spew forth lava which forms new rocks on top of old, in a few cases build new land out in the sea. The case of a completely new volcano rising from the sea and starting a new island is very, very rare. That said, even an incremental contribution to knowledge can be extremely valuable and form the basis for further study either as postdoctoral work or as future PhDs for other people. If your PhD is examined at an oral examination (often referred to as the viva, short for viva voce), then possibly the most likely question of all to be asked is "What do you consider to be the most important contribution to knowledge of your study?"
What Have You Learnt?
As an examiner and a supervisor, I take the view that a PhD is still a learning exercise for the student. I always encourage submitting students or candidates at examination to reflect upon the question "What have you learnt during the process of doing the PhD?" It may be knowledge about the problem under scrutiny, but it could equally well be knowledge about the research methodology. In addition, it may be knowledge which is of general interest or it may be personal insights or personal development. As well as having the goal of making a contribution to knowledge, a PhD programme is designed to turn out graduates capable of being independent researchers, and possibly starting to train others in research. In order to be competent as independent researchers, I think that those completing PhDs need to show a degree of self awareness and reflection that will allow them to operate autonomously. This includes knowledge of their limits both as a researcher and of their research methods and tools. It also includes an appreciation of those limits imposed by ethical considerations. A PhD is often view as an entry ticket into a career as a professional researcher and any professional is required to know their own limits and work within them.
Revisiting Your Objectives
A good way to structure your PhD thesis conclusion is to revisit your stated objectives from the start of the thesis. You can review how well you have met each in turn: What did you learn from the literature review? How effective was your study design? How well did your data collection go? Did you get the results you expected? In each case, where the outcome was not as expected, there is scope for reflection and adding value by exploring why you obtained the outcome you did. This is an opportunity to turn a "failure" into a positive contribution by showing you have learnt from the experience.
Evidencing Your Conclusions
All parts of your thesis should be evidenced. All the way through your PhD narrative, the reasons why you did things are as important as what you did. However, the conclusions are not a place to introduce new evidence. Your conclusions should be evidenced by referring back to other parts of the thesis where the evidence may either be supplied from the literature or from your own study, or ideally by comparing both. For example, you might have a conclusion along the lines of "In her paper reviewed in Chapter n, X (2005) states that… In my own study, as described in Chapter m, I found that …The reason for this difference may be that X did this and I did that."
The Importance Of Your Conclusions
I maintain that the conclusions are the most important part of your thesis. A good literature review, a solid study design, an effective presentation of your results are all necessary but insufficient to obtain a PhD degree. The principal criterion for the award of the degree is making a contribution to knowledge. This is expressed through the conclusions which therefore are crucial to a successful outcome.