Rule of Three: advice on writing a PhD thesis

PhD students sometimes get the same bad advice on writing their thesis. I call this advice the Rule of Three. Typically, they get told that their thesis:

  • Will take 3 months to write
  • Should have 3 results chapters
  • Should be 300 pages

These bits of advice have one thing in common: they are all wrong.

  • If you have been organised (see below), it should not take 3 months to write a PhD thesis. It certainly shouldn’t involve leaving the lab 3 months before your hand-in date to write up.
  • Theses can have one results chapter or they can have more. How many chapters depends on your project, and your results. Trying to make three results chapters out of one chapter ends up in a weak or overlong thesis.
  • A thesis is like a piece of string and it will be as long as it needs to be. Aim for brevity and not producing a magnus opus (see below).

Disclaimer: what follows is some different advice. As with all “advice”, your mileage will vary. It is written for the people in my lab but likely applies to UK PhD students doing biomedical research.

Rule 1: aim for a thesis that is good enough

Who will read your thesis? Two people. Your examiners. OK, some parts – such as the Methods section – will be useful to future lab members (although with electronic lab notebooks this function is becoming redundant). Maybe your thesis will be downloaded by someone from the repository, but essentially, it will only be read by your examiners.

How long does an examiner spend reading your thesis? A few hours. One day maximum. They simply have no more time. Do you really want to spend three months of your life writing something that will be read for just a few hours by two people?

It’s for these reasons that spending too much effort writing a perfect thesis is a waste of time. It just needs to be good enough.

As well as being just good enough, it only needs to be long enough. A big mistake students make is to produce a really long thesis because they think that that is what theses should be (rule of 3). What happens is the examiner will receive the thesis, look at how many pages there are, subtract the bibliography, and their heart will sink if it is too long.

You might now be wondering: is writing a thesis a waste of time?

No, because you have to do it to get your PhD.

No, because you learn important writing skills. You also learn how to assemble a large document (it’s often how students learn to use Word properly or up their LaTeX game). It’s good training for writing papers and other technical documents down the line. Employers know this when they hire you.

But that is about it. So you just need to write something that is good enough to pass.

Rule 2: prioritise papers and the thesis will follow

Papers are the priority. They are more useful to you and to your PI. But this advice isn’t motivated by self-interest. If you go into the viva and the work in your thesis is already peer reviewed and published, it’s harder for the examiners to criticise it. At least, they will not approach your thesis with the question: is this work publishable? This is one criteria for passing your PhD, so demonstrating that it is publishable means you are (almost) there.

This was the one bit of advice I received when doing my PhD and it is still true today. OK, it is harder these days to get a first author paper published before you submit your thesis. However a preprint on bioRxiv before you begin writing will help you to prepare your thesis and will still tick the publishable box.

How long should it take to write your thesis?

There is tension here because you are at your most useful in the lab as you near the end of your PhD. One week of labwork now is worth one month (or more) earlier in your PhD. You are most valuable to your lab/PI/science/career at this point and keeping working in the lab will yield more rewards. But it won’t get your thesis written.

The first bit of writing is busywork and can be done around lab work. “Deep writing” and reading does need time away for most students.

If you have only collected data in the lab and not analysed it, if you’ve not presented your work very often, if you are disorganised… yes, it will take you a full three months to write your thesis.

All the folks in my lab are encouraged to get figures ready, analyse as they go and they also give regular talks. It should not take anyone in this position three months away from the lab to write their thesis.

Agreeing a timeline with your PI for when you begin writing is really important. Regular deadlines and a commitment to timely feedback from your supervisor make thesis writing easier. The discussion needs to be based on facts though. Often students want to budget a lot of time to writing, because of the rule of 3 or because they believe they are “bad at writing”. It helps to see some evidence. Writing draft chapters earlier in the PhD – which is a requirement at some universities – can reveal difficulties and weaknesses.

Reality check

If you hear the rule of 3 from everyone and your supervisor is giving you different advice. It might be time for a reality check. Have a look at past theses from the lab. How long were they? How many chapters? Information is good.

YearResults ChaptersPages excl. bibliography/appendicesPapers associated with thesis
201132081 published paper. Other work published later.
201322031 published paper. Appendix contains a further paper and methods paper.
201631331 published, 1 in preparation.
20163941 paper in revision.
201721381 paper submitted. Contributed to three other papers
201831212 papers (but not published or submitted at the time). Appendix with one paper they contributed to.
Previous theses from the lab

You can see that all theses are fewer than 300 pages in length, many substantially so. Four have three chapters and two have two. Although looking closer, two of the theses with three chapters use a results chapter as an expanded methods chapter.

Ultimately, the thesis is your work but you will get input from your supervisor. Regardless of what is written here or how many people tell you about the rule of 3, your supervisor will have their own ideas about how your thesis should be. Agreeing a sensible plan with them is the way to get started productively.

Getting started

This is not a comprehensive guide but in order to write a good enough thesis, you first need a plan.

  • Make a figure list. This should be every single figure you can think of. You can cross off ones you don’t need later if they don’t fit or are insignificant.
  • Plan the narrative. There is usually more than one way to put together the figures to make a thesis. Be prepared for this to change after you start writing! Sometimes the writing process reveals ways in which the narrative should be rearranged.
  • How many results chapters? Start with the idea that you will have one. Does it need dividing? If yes, then what are the titles of the two chapters? If you have difficulty titling them you may need to split to a 3rd.

Now you have a plan. It’s time to get going.

Set some goals – but make them small. Having a goal of “I am going to complete my thesis” is too demoralising. You need to feel like you are making progress constantly to stay motivated. Break it down into smaller chunks. “I will finish this chapter by next Friday”. “I will write the cloning section this morning and then go for a walk”.

Write the materials and methods first. It’s the easiest bit to write because it is all technical writing with little wordcraft required. You can fit it around labwork. In fact, it is easier to write whilst in the lab because you can look up all the stuff you need. Importantly, it gets over the “blank page syndrome”.

Next get your figures together. This should already be done if you have been organised.

Then write the figure legends. You already have the title for each figure from your plan. All you need to do is describe each panel. Again, quite low energy writing required for this task.

Now write the results sections! This is the same way that we put papers together. The results parts of the thesis are more extended but in principle you will guide the reader though the figures that you’ve made. Remember, you already have the legends written. So you are already partly on your way.

Time to regroup. At this point you can refine your plan for the introduction and check the rest of your plan still makes sense. Now is the time for some deep writing and reading.

Good luck!

The post title comes from “Rule of Three” by The Lemonheads.

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