“Don’t use the same word twice.”

There’s always that memory that you have from your childhood that still influences your life today. When I was probably 8 years old, someone showed me a centipede on the ground and then said, “Don’t touch it. It’s poisonous.” Of course, now I know better, but I still can’t help but be unrealistically reluctant today.

After years of teaching scientific writing, I’ve come to discover another thing we learned in elementary school during English composition class that still affects the way that many of us write our science: “Don’t use the same word twice.”

I’ve asked students in my workshops if they heard of this “rule” before. They have, and it was early in their education when they first heard it. I’m surprised that this rule is promoted even in different cultures and countries. It seems many of us learned to write through using this rule. And, you can find websites that still promote this rule.

At the time, such writing advice probably made sense. You can imagine the frustration of elementary school teachers whose students always fall back on the same words, repeating and repeating those words until they are beat into the ground.

Of course, novelists and other professional writers tend to vary their language naturally because it makes the writing more interesting for the reader, especially in creative writing styles.

But, this “rule” needs to be reconsidered for scientific writing. Science is hard enough to read and understand, but if we keep changing the words with the same meaning from sentence to sentence, then we make it difficult for the reader to follow. Consider the following two sentences from a recent paper.

A total of 10,584 participants entered their demographic information and at least one pain report, making them eligible for the present study. A total of 6,850 (65%) participants remained in the study beyond their first week and 4,692 (44%) beyond their first month. Even after 200 days, 15% of participants were still entering data nearly every day (Druce et al. 2017; Fig. 2b in Dixon et al. 2019). [Emphasis added.]

I’ve reused the word participants three times in three consecutive sentences. To vary the wording, I could have used respondents, patients, or citizen scientists, but that might have made the reader question what the difference between a participant and a patient was. Even if the reader was clear that the words meant exactly the same thing in this context, there’s a microsecond of mental energy spent doing what didn’t need to be done if the same word was just reused.

We scientists and engineers have defined terminology for precision of expression. If I use the term quasigeostrophic potential vorticity, there are no synonyms that can be employed. So, the term has to be used repeatedly in the same sentence, if need be, to ensure precision of expression.

I do want to make it clear that this doesn’t apply to the more descriptive aspects of the text. For example, you don’t have to keep saying “Figure X shows” sentence after sentence. You can vary shows with presents, illustrates, or displays. Or alternatively you can rearrange the sentence to describe the science rather than the figure.

The same applies to sentence structures. Repeating sentence structures helps readers see the relationships and what has changed from one sentence to another. Consider this quote from the same paper.

Specifically, our results indicate that the top 10% of days with a high percentage of participants (about 20%) experiencing a pain event (represented here by a +1 change or greater in their pain level on a 5-point scale; referred to as a high-pain day) were associated with below-normal pressure, above-normal humidity, higher precipitation rate, and stronger wind. In contrast, the bottom 10% of days with a small percentage of participants (about 10%) experiencing a pain event (a low-pain day) were associated with above-normal pressure, below-normal humidity, lower precipitation rate, and weaker wind.

The two sentences presented here have nearly the exact same structure, but just present the two different groupings of data studied (i.e., the top 10% of days and the bottom 10% of days) and their results. Using the same sentence structure allows readers to more easily compare the difference between these two groupings. Think of it like a controlled experiment (only varying one variable or quantity from one experiment to the next to examine the sensitivity to changing only one variable), but for your writing.

You learned a lot of useful information early in your education, lessons that you still carry to this day. Many are useful. This “rule” of not repeating words or phrases needs to be re-evaluated when you write science.