How To Write A Dissertation
Bedtime Reading For People Who Do Not Have Time To Sleep
To The Candidate:
So, you are preparing to write a Ph.D. dissertation in an experimental area
of Computer Science. Unless you have written many formal documents before, you
are in for a surprise: it's difficult!
There are two possible paths to success:
- Planning Ahead.
Few take this path. The few who do leave the University so quickly that
they are hardly noticed. If you want to make a lasting impression and have a
long career as a graduate student, do not choose it.
All you really have to do is outlast your doctoral committee. The good
news is that they are much older than you, so you can guess who will
eventually expire first. The bad news is that they are more practiced at
this game (after all, they persevered in the face of their doctoral
committee, didn't they?).
Here are a few guidelines that may help you when you finally get serious
about writing. The list goes on forever; you probably won't want to read it all
at once. But, please read it before you write anything.
The General Idea:
- A thesis is a hypothesis or conjecture.
- A PhD dissertation is a lengthy, formal document that argues in defense of
a particular thesis. (So many people use the term ``thesis'' to refer to the
document that a current dictionary now includes it as the third meaning of
- Two important adjectives used to describe a dissertation are ``original''
and ``substantial.'' The research performed to support a thesis must be both,
and the dissertation must show it to be so. In particular, a dissertation
highlights original contributions.
- The scientific method means starting with a hypothesis and then collecting
evidence to support or deny it. Before one can write a dissertation defending
a particular thesis, one must collect evidence that supports it. Thus, the
most difficult aspect of writing a dissertation consists of organizing the
evidence and associated discussions into a coherent form.
- The essence of a dissertation is critical thinking, not experimental data.
Analysis and concepts form the heart of the work.
- A dissertation concentrates on principles: it states the lessons learned,
and not merely the facts behind them.
- In general, every statement in a dissertation must be supported either by
a reference to published scientific literature or by original work. Moreover,
a dissertation does not repeat the details of critical thinking and analysis
found in published sources; it uses the results as fact and refers the reader
to the source for further details.
- Each sentence in a dissertation must be complete and correct in a
grammatical sense. Moreover, a dissertation must satisfy the stringent rules
of formal grammar (e.g., no contractions, no colloquialisms, no slurs, no
undefined technical jargon, no hidden jokes, and no slang, even when such
terms or phrases are in common use in the spoken language). Indeed, the
writing in a dissertaton must be crystal clear. Shades of meaning matter; the
terminology and prose must make fine distinctions. The words must convey
exactly the meaning intended, nothing more and nothing less.
- Each statement in a dissertation must be correct and defensible in a
logical and scientific sense. Moreover, the discussions in a dissertation must
satisfy the most stringent rules of logic applied to mathematics and science.
What One Should Learn From The Exercise:
- All scientists need to communicate discoveries; the PhD dissertation
provides training for communication with other scientists.
- Writing a dissertation requires a student to think deeply, to organize
technical discussion, to muster arguments that will convince other scientists,
and to follow rules for rigorous, formal presentation of the arguments and
A Rule Of Thumb:
Good writing is essential in a dissertation. However, good writing cannot
compensate for a paucity of ideas or concepts. Quite the contrary, a clear
presentation always exposes weaknesses.
Definitions And Terminology:
- Each technical term used in a dissertation must be defined either by a
reference to a previously published definition (for standard terms with their
usual meaning) or by a precise, unambiguous definition that appears before the
term is used (for a new term or a standard term used in an unusual way).
- Each term should be used in one and only one way throughout the
- The easiest way to avoid a long series of definitions is to include a
statement: ``the terminology used throughout this document follows that given
in [CITATION].'' Then, only define exceptions.
- The introductory chapter can give the intuition (i.e., informal
definitions) of terms provided they are defined more precisely later.
Terms And Phrases To Avoid:
Mostly, they are very often overly used. Use strong words instead. For
example, one could say, ``Writers abuse adverbs.''
- jokes or puns
They have no place in a formal document.
- ``bad'', ``good'', ``nice'', ``terrible'', ``stupid''
A scientific dissertation does not make moral judgements. Use
``incorrect/correct'' to refer to factual correctness or errors. Use precise
words or phrases to assess quality (e.g., ``method A requires less
computation than method B''). In general, one should avoid all qualitative
- ``true'', ``pure'',
In the sense of ``good'' (it is judgemental).
- ``an ideal solution''
- ``today'', ``modern times''
Today is tomorrow's yesterday.
How soon? Later tonight? Next decade?
- ``we were surprised to learn...''
Even if you were, so what?
- ``seems'', ``seemingly'',
It doesn't matter how something appears;
- ``would seem to show''
all that matters are the facts.
- ``in terms of''
- ``based on'', ``X-based'', ``as the basis of''
Does not mean ``various''; different than what?
- ``in light of''
- ``lots of''
- ``kind of''
- ``type of''
- ``something like''
- ``just about''
- ``number of''
vague; do you mean ``some'', ``many'', or ``most''? A quantative
statement is preferable.
- ``due to''
only if you know the statistical probability (if you do, state it
- ``obviously, clearly''
be careful: obvious/clear to everyone?
Can have a negative connotation, as in ``simpleton''
- ``along with''
- ``actually, really''
define terms precisely to eliminate the need to clarify
- ``the fact that''
makes it a meta-sentence; rephrase
- ``this'', ``that''
As in ``This causes concern.'' Reason: ``this'' can refer to the subject
of the previous sentence, the entire previous sentence, the entire previous
paragraph, the entire previous section, etc. More important, it can be
interpreted in the concrete sense or in the meta-sense. For example, in:
``X does Y. This means ...'' the reader can assume ``this'' refers
to Y or to the fact that X does it. Even when restricted (e.g., ``this
computation...''), the phrase is weak and often ambiguous.
- ``You will read about...''
The second person has no place in a formal dissertation.
- ``I will describe...''
The first person has no place in a formal dissertation. If
self-reference is essential, phrase it as ``Section 10 describes...''
- ``we'' as in ``we see that''
A trap to avoid. Reason: almost any sentence can be written to begin
with ``we'' because ``we'' can refer to: the reader and author, the author
and advisor, the author and research team, experimental computer scientists,
the entire computer science community, the science community, or some other
- ``Hopefully, the program...''
Computer programs don't hope, not unless they implement AI systems. By
the way, if you are writing an AI thesis, talk to someone else: AI people
have their own system of rules.
- ``...a famous researcher...''
It doesn't matter who said it or who did it. In fact, such statements
prejudice the reader.
- Be Careful When Using ``few, most, all, any, every''.
A dissertation is precise. If a sentence says ``Most computer systems
contain X'', you must be able to defend it. Are you sure you really know the
facts? How many computers were built and sold yesterday?
- ``must'', ``always''
- ``proof'', ``prove''
Would a mathematician agree that it's a proof?
Used in the sense of ``prove''. To ``show'' something, you need to
provide a formal proof.
Your mother probably told you the difference.
Use active constructions. For example, say ``the operating system starts
the device'' instead of ``the device is started by the operating system.''
Write in the present tense. For example, say ``The system writes a page to
the disk and then uses the frame...'' instead of ``The system will use the
frame after it wrote the page to disk...''
Define Negation Early:
Example: say ``no data block waits on the output queue'' instead of ``a
data block awaiting output is not on the queue.''
Grammar And Logic:
Be careful that the subject of each sentence really does what the verb
says it does. Saying ``Programs must make procedure calls using the X
instruction'' is not the same as saying ``Programs must use the X instruction
when they call a procedure.'' In fact, the first is patently false! Another
example: ``RPC requires programs to transmit large packets'' is not the same
as ``RPC requires a mechanism that allows programs to transmit large
All computer scientists should know the rules of logic. Unfortunately the
rules are more difficult to follow when the language of discourse is English
instead of mathematical symbols. For example, the sentence ``There is a
compiler that translates the N languages by...'' means a single compiler
exists that handles all the languages, while the sentence ``For each of the N
languages, there is a compiler that translates...'' means that there may be 1
compiler, 2 compilers, or N compilers. When written using mathematical
symbols, the difference are obvious because ``for all'' and ``there exists''
Focus On Results And Not The People/Circumstances In Which They Were
``After working eight hours in the lab that night, we realized...'' has no
place in the dissertation. It doesn't matter when you realized it or how long
you worked to obtain the answer. Another example: ``Jim and I arrived at the
numbers shown in Table 3 by measuring...'' Put an acknowledgement to Jim in
the dissertation, but do not include names (even your own) in the main body.
You may be tempted to document a long series of experiments that produced
nothing or a coincidence that resulted in success. Avoid it completely. In
particular, do not document seemingly mystical influences (e.g., ``if that cat
had not crawled through the hole in the floor, we might not have discovered
the power supply error indicator on the network bridge''). Never attribute
such events to mystical causes or imply that strange forces may have affected
your results. Summary: stick to the plain facts. Describe the results without
dwelling on your reactions or events that helped you achieve them.
Avoid Self-Assessment (both praise and criticism):
Both of the following examples are incorrect: ``The method outlined in
Section 2 represents a major breakthrough in the design of distributed systems
because...'' ``Although the technique in the next section is not
References To Extant Work:
One always cites papers, not authors. Thus, one uses a singular verb to
refer to a paper even though it has multiple authors. For example ``Johnson
and Smith [J&S90] reports that...''
Avoid the phrase ``the authors claim that X''. The use of ``claim'' casts
doubt on ``X'' because it references the authors' thoughts instead of the
facts. If you agree ``X'' is correct, simply state ``X'' followed by a
reference. If one absolutely must reference a paper instead of a result, say
``the paper states that...'' or ``Johnson and Smith [J&S 90] presents
Concept Vs. Instance:
A reader can become confused when a concept and an instance of it are
blurred. Common examples include: an algorithm and a particular program that
implements it, a programming language and a compiler, a general abstraction
and its particular implementation in a computer system, a data structure and a
particular instance of it in memory.
Terminology For Concepts And Abstractions
When defining the terminology for a concept, be careful to decide
precisely how the idea translates to an implementation. Consider the following
VM systems include a concept known as an address space. The system
dynamically creates an address space when a program needs one, and destroys an
address space when the program that created the space has finished using it. A
VM system uses a small, finite number to identify each address space.
Conceptually, one understands that each new address space should have a new
identifier. However, if a VM system executes so long that it exhausts all
possible address space identifiers, it must reuse a number.
The important point is that the discussion only makes sense because it
defines ``address space'' independently from ``address space identifier''. If
one expects to discuss the differences between a concept and its
implementation, the definitions must allow such a distinction.
Knowledge Vs. Data
The facts that result from an experiment are called ``data''. The term
``knowledge'' implies that the facts have been analyzed, condensed, or
combined with facts from other experiments to produce useful information.
Cause and Effect:
A dissertation must carefully separate cause-effect relationships from
simple statistical correlations. For example, even if all computer programs
written in Professor X's lab require more memory than the computer programs
written in Professor Y's lab, it may not have anything to do with the
professors or the lab or the programmers (e.g., maybe the people working in
professor X's lab are working on applications that require more memory than
the applications in professor Y's lab).
Drawing Only Warranted Conclusions:
One must be careful to only draw conclusions that the evidence supports.
For example, if programs run much slower on computer A than on computer B, one
cannot conclude that the processor in A is slower than the processor in B
unless one has ruled out all differences in the computers' operating systems,
input or output devices, memory size, memory cache, or internal bus bandwidth.
In fact, one must still refrain from judgement unless one has the results from
a controlled experiment (e.g., running a set of several programs many times,
each when the computer is otherwise idle). Even if the cause of some
phenomenon seems obvious, one cannot draw a conclusion without solid,
Commerce and Science:
In a scientific dissertation, one never draws conclusions about the
economic viability or commercial success of an idea/method, nor does one
speculate about the history of development or origins of an idea. A scientist
must remain objective about the merits of an idea independent of its
commercial popularity. In particular, a scientist never assumes that
commercial success is a valid measure of merit (many popular products are
neither well-designed nor well-engineered). Thus, statements such as ``over
four hundred vendors make products using technique Y'' are irrelevant in a
Politics And Science:
A scientist avoids all political influence when assessing ideas.
Obviously, it should not matter whether government bodies, political parties,
religious groups, or other organizations endorse an idea. More important and
often overlooked, it does not matter whether an idea originated with a
scientist who has already won a Nobel prize or a first-year graduate student.
One must assess the idea independent of the source.
In general, every dissertation must define the problem that motivated the
research, tell why that problem is important, tell what others have done,
describe the new contribution, document the experiments that validate the
contribution, and draw conclusions. There is no canonical organization for a
dissertation; each is unique. However, novices writing a dissertation in the
experimental areas of CS may find the following example a good starting point:
Chapter 1: Introduction
An overview of the problem; why it is important; a summary of extant
work and a statement of your hypothesis or specific question to be
explored. Make it readable by anyone.
Chapter 2: Definitions
New terms only. Make the definitions precise, concise, and
Chapter 3: Conceptual Model
Describe the central concept underlying your work. Make it a ``theme''
that ties together all your arguments. It should provide an answer to the
question posed in the introduction at a conceptual level. If necessary,
add another chapter to give additional reasoning about the problem or its
Chapter 4: Experimental Measurements
Describe the results of experiments that provide evidence in support
of your thesis. Usually experiments either emphasize proof-of-concept
(demonstrating the viability of a method/technique) or efficiency
(demonstrating that a method/technique provides better performance than
those that exist).
Chapter 5: Corollaries And Consequences
Describe variations, extensions, or other applications of the central
Chapter 6: Conclusions
Summarize what was learned and how it can be applied. Mention the
possibilities for future research.
A short (few paragraphs) summary of the the dissertation. Describe the
problem and the research approach. Emphasize the original contributions.
Suggested Order For Writing:
The easiest way to build a dissertation is inside-out. Begin by writing
the chapters that describe your research (3, 4, and 5 in the above outline).
Collect terms as they arise and keep a definition for each. Define each
technical term, even if you use it in a conventional manner.
Organize the definitions into a separate chapter. Make the definitions
precise and formal. Review later chapters to verify that each use of a
technical term adheres to its definition. After reading the middle chapters to
verify terminology, write the conclusions. Write the introduction next.
Finally, complete an abstract.
Key To Success:
By the way, there is a key to success: practice. No one ever learned to
write by reading essays like this. Instead, you need to practice, practice,
practice. Every day.
We leave you with the following ideas to mull over. If they don't mean
anything to you now, revisit them after you finish wirting a dissertation.
After great pain, a formal feeling comes.
A man may write at any time,
if he will set himself doggedly to it.
Keep right on to the end of the
The average Ph.D. thesis is
nothing but the transference of bones from one graveyard to another.