How to write a limitations section

Many researchers confess that the limitations section is the hardest to write. No one wants to admit that their study has limitations. After all, why would a scientist want to publish a limited paper? Nevertheless, the way in which an author acknowledges a paper’s limitations can make the difference between a good paper and a great paper.

Three types of limitations

The best time to think about a paper’s limitations is not when you are finishing the writing, but rather when you are designing the study! You never want to smack yourself in the forehead during a study and wonder why you hadn’t thought of something before you started this project. You certainly do not want reviewers to point out limitations that you hadn’t already thought of.

Design limitations

Say you are studying the effect of gamma rays on man-in-the-moon marigolds. You design an experiment in which you artificially expose the marigolds to gamma rays in a laboratory setting. The advantage is that you can control the dose of gamma rays as well as various parameters related to marigold growth. What you cannot do is reproduce the real-life conditions of the gamma-ray exposure of marigolds in a garden. Your paper has a design limitation. It is best to acknowledge this limitation ahead of time and plan to mention it in your discussion.

Impact limitations

Because you studied only man-in-the-moon marigolds, you cannot say for sure whether the effects you measured were generalisable to, say, Mexican marigolds or Southern Cone marigolds. This is called an ‘impact limitation’. It is essentially a problem of generalisability. A colleague reading your paper may want to know whether your findings are relevant to the research he or she is doing.

Statistical or data limitations

These are the types of limitations that most often appear in academic journals. Generally, they take the form of an investigator acknowledging that his or her sample size was too small to generate statistical significance. Perhaps there was a freak accident in which rabbits escaped from the animal care facility and ate half your marigolds, severely fouling up your power calculation. You should acknowledge this kind of limitation at the end of your paper.

Brick wall

Many researchers confess that the limitations section is the hardest to write.

How to write about limitations

A proper limitations paragraph has three parts: the announcement, the reflection and the look forward.

The announcement

This is the part most investigators are good at writing. For example: ‘This study has several limitations. First, the number of marigolds studied was small because of the unfortunate rabbit incident…’ Do not forget that statistical limitations are only one of three types. You should also make an announcement regarding the study design flaws and non-generalisability concerns.

The reflection

The best limitations sections include careful justifications for the choices you made during the research process. ‘We felt that simulated laboratory conditions were the only way in which to measure gamma-ray doses and marigold exposure accurately.’ Reflections such as these demonstrate to the editor and reviewers that you thought genuinely about the limitations of your investigation and made the best choices to achieve the desired objectives.

The look forward

The last part of the limitations section is where you tell the reader what you plan to do to overcome the limitations, or what future questions and research avenues are opened, thanks to your study: ‘It remains an open question as to whether real-life gamma-ray exposure has similar effects on man-in-the-moon marigolds in gardens or greenhouses’.

Take-home points

• Think hard about your study’s limitations during the planning process.
• Don’t neglect study design and impact/generalisability limitations.
• Remember: announce, reflect, look forward.

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