Writing Clear Comparisons in Scientific Studies

Whether you are reporting an experimental study, proposing a new method, or presenting a literature review, chances are you will be including sentences that involve comparisons. Although establishing comparisons is common in scientific research, writing sentences that clearly describe these comparisons can be challenging. In this article, we point out common mistakes in writing comparisons as well as illustrate how to write precise comparative sentences.

What is a comparison in scientific writing?

A comparison examines the relationship between two or more things to determine similarities or differences between them. Comparisons are a common rhetorical strategy for organizing ideas and making meaning, and they are an important component of scientific writing. For example, when we set up an experiment, we often have more than one group, such as an experimental group and a control group. We might compare patients with diabetes and healthy individuals, cells treated with a tyrosine kinase inhibitor and untreated cells, or transgenic plants and regular plants. Additionally, when we propose a new method or design, we often compare its performance to existing methods. A literature review may also involve comparisons, such as comparing the results of one study with that of a subsequent study. Importantly, the strategy of comparison also involves contrasts, that is, focusing on differences.

Below we present three basic guidelines to follow to ensure your comparative sentences are precise and complete.

1. Make sure to include all parts of the comparison.

A frequent error in writing comparative sentences is omitting part of the comparison (i.e., failing to complete the comparison). Take a look at the examples below.

Incomplete: Tumor cells treated with compound A showed lower expression of Bcl2.
Complete: Tumor cells treated with compound A showed lower expression of Bcl2 than vehicle control-treated cells.

In the first sentence we don’t know what the tumor cells treated with compound A are being compared with. This statement begs the question, “They showed lower expression of Bc12 compared with what? Or “Lower than what?”

The second sentence includes the two types of cells being compared. Note that the comparative adjective “lower” is used. Comparative adjectives are words such as “higher,” “greater,” “fewer,” and “smaller,” that indicate the difference in the relationship between things. These words will often be accompanied by the word “than” in comparative sentences.

Incomplete: Our proposed algorithm is a more robust and accurate method for predicting erosion in pipelines.
Complete:
Our proposed algorithm is more robust and accurate for predicting erosion in pipelines than existing algorithms.
Compared with existing algorithms, our proposed algorithm is more robust and accurate for predicting erosion in pipelines.
Our proposed algorithm is a robust and accurate method for predicting erosion in pipelines.

In this example, if we use “more robust and accurate,” we need to say what is compared with our algorithm. Alternatively, removing the word “more” makes the sentence grammatically correct. As a conclusion of the study, we may only want to say that our method is robust and accurate without making a comparison.

Acceptable: A better understanding of the pathogenesis of muscular dystrophy may lead to novel therapies.

Occasionally we see sentences like the example above in manuscripts. The sentence here does not explicitly indicate the two items being compared because it is understood that the authors mean an understanding better than what we currently know about the pathogenesis of the disease. Such sentences should be used sparingly in a manuscript and never be used to describe specific experimental results.

2. Make sure the items are comparable.

We can only compare things of the same type. In the examples above, cells are compared with other cells, and an algorithm is compared with other algorithms. In our experiments, we of course only compare things of the same type, but if we are not careful when we write up the results, readers may get confused regarding what is compared with what.

Incorrect: The 1000-grain weight of rice treated with fertilizer A was higher than fertilizer B.
Incorrect: The 1000-grain weight of rice treated with fertilizer A was higher than rice treated with fertilizer B.
Correct: The 1000-grain weight of rice treated with fertilizer A was higher than that of rice treated with fertilizer B.
Correct: Rice treated with fertilizer A had a higher 1000-grain weight than rice treated with fertilizer B.


In this example, the 1000-grain weight is being compared, but in the two incorrectly written sentences, it is compared with either a fertilizer or rice. One way to solve the problem is to add “that of” in the sentence. We can also restructure the sentence as shown in the last sentence of the example.

3. Choose precise words for changes over time and differences between groups.

Words such as “increase” and “decrease” describe a trend and thus imply changes in the same sample over time. To describe differences between distinct groups, comparative adjectives such as “greater,” “larger,” “lower,” “smaller,” and “shorter” should be used. Imagine we have a study where patients with low hematocrit received two different treatments, and hematocrit levels were measured in the two groups of patients before and at different time points after the treatments. When describing the results, it is necessary to be clear whether we mean changes in the same group or differences between the two groups.

Describing a trend in each group: Hematocrit levels increased in both groups following the treatments and peaked at 1 week post-treatment in group A but peaked at 3 weeks post-treatment in group B.
Describing a between-group comparison: At the 3-month post-treatment follow-up, patients in group A had higher hematocrit levels than those in group B.

In the first sentence, the words “increased” and “peaked” describe a trend over time that took place in each group. In the second sentence, “higher than” describes a contrast between the groups.

Clarity is key in scientific writing. Following the three basic guidelines discussed here will ensure your comparative sentences leave nothing for the reader to fill in on their own.