Can section numbering depth exceed table of content depth in thesis?

Question

I’m writing up my thesis and use two levels of structure in my table of contents, which looks something like this:

Table of contents

1. Introduction

   1.1 Subsection intro

2. Methods

   2.1 Methods subsection

   2.2 Etcetera

However, the section numbering in my thesis has three levels, so for example section 2.1 contains a subsection 2.1.1

2.1 Methods subsection

blablabla

2.1.1 Methods subsubsection

More bla bla bla

I don’t want to have three levels in my table of contents because that really clutters it up. However, I don’t want to reduce the number of actual levels to two because I refer to the subsubsection in my text as well. Is this OK or is there some convention that says you have to have the same numbering depth in your table of contents and text?

Answer 1

Is this OK or is there some convention that says you have to have the same numbering depth in your table of contents and text?

There is no general convention against this, but your supervisor or university may hold dogmatic views on it, so you have ask them or read their regulations, respectively.

That being said, the question whether it’s a good idea can usually be answered best if you consider the purpose of the respective elements:

• The purpose of the table of contents is to allow the reader to quickly find a specific part of your text and to give a rough overview over your structure. You do not need a certain subsubsection or similar in your table of contents, if both:

  • it is safe to assume that everybody who is looking for the contents of some subsubsection or similar will find them as well if it is not explicitly listed in the table of contents;

  • the subsubsection is not essential to your thesis’ structure (which is very likely).

  Actually, I think that many theses do not need a table of contents that goes beyond the first level.

• The purpose of having subsubsections (and similar) is to structure your text and to be able to have precise references within the text (e.g., “see Sec. 4.3.2“). Thus a subsubsection is not needed, if both:

  • You do not specifically reference it.
  • It does not help the reader to see how your content is structured.

Answer 2

Unless (as @schester says) you have some guidelines or a supervisor that says "this is not okay", I don't think there is any convention that forbids this.

But, I would ask myself two questions:

• do I really need that many levels of subsections?
• or, does it really clutter things up that much to have three levels in the table of contents?

I think the answer to both of those questions depends very much on your actual thesis. In my MSc thesis I did have three levels of subsections in the table-of-contents, but only two of the sections had subsubsections.

What I would finally point out is that a long table of contents is not necessarily a bad thing. I have a very classic¹ textbook on my desk right now that has a 9 page table of contents, and I love that.

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¹ This book was first published in 1879 and is still in active use in my field, the latest edition is from 2006. Yep, it's "Hydrodynamics" by Sir Horace Lamb.