Using tenses in scientific writing
Tense considerations for science writing
When you write an experimental report, or draft a thesis chapter, you need to choose which tense, or tenses, to use. This flyer provides advice intended to help you become more conscious of what the choice of verb tense involves, and to become better able to notice the tense choices that writers in your particular field have made.
From your chosen tense, your reader receives two kinds of information. One concerns time: it is about ‘when’ (past, present or future). The other relates to whether an event or process is open or closed.
The examples below illustrate the distinction between ‘open’ and ‘closed’ events:
How long were you at Melbourne Uni?
Simple past / closed event: the other person has already graduated.
How long have you been at Melbourne Uni?
Present perfect / open event: the other person is still engaged on his / her course.
What do tenses do?
Verb tenses present a relationship between
• the present moment (now), and,
• another moment or period in time (which may be
long or short).
These moments or periods may be in the past, present or future.
Tenses manage time by placing them within particular relationships or ‘time frameworks’.
As a generalisation: in various types of scientific writing, some time frameworks are more commonly used than others. Their frequency varies from one section of a paper or report to another, and they can also vary between one scientific discipline and another.
The next section gives some advice about the various tenses.
This usually refers to your unpublished results and uses the past tense.
Your introduction needs to include background information which is generally accepted as fact in a discipline. You also need to explain why the research you are reporting is important. It is usually presented in the present tense. Example:
Genomics provides crucial information for rational drug design.
You will need to refer to existing research relevant to your work, and you can indicate your opinion of the research you are writing about by careful tense selection.
For example, when you use the present tense you are indicating to the reader that you believe that the research findings are still true and relevant, even though the original research may have been conducted some
time ago. Example:
Many of the lakes and wetlands in the region are located in craters or valleys blocked by early Pliocene lava flows (Ollier & Joyce, 1964).
Present perfect tense to report research
If you use present perfect tense in your introduction when you refer to previous research, you communicate ‘recency’ or ‘currency’. Currency may be positive (asserting that previous studies have established a firm research foundation) or negative (asserting that not enough relevant or valid work has yet been done).
Positive and negative currency can even be asserted in the same sentence, as in the example below (which uses the passive voice): Example:
A great deal of research has been conducted on the basic techniques of nuclear transfer, but few experiments have been carried out to discover the most appropriate age of the cytoplasm to support nuclear transfer most effectively.
This suggests that you believe that more experiments are necessary. The existence of a ‘research gap’ is further emphasised by the phrase: ‘but few experiments’.
Past tense to describe what was done
In your methods section it is customary to use a form of the simple past tense to describe what you did in your study. Passive voice is often used. Examples:
Total phosphorous (TP) and total nitrogen (TN) were measured in the laboratory using standard procedures.
The standard protocol was followed for the preparation of the media from stock solutions.
The two previous examples are in the past tense, but in the passive voice. Have a look at the following examples in the past passive and past active voice.
Three 2 litre samples were taken at a depth of between 0.1 and 0.5 m at the down-wind end of each wetland.
Each of the three groups took 2 litre samples at a depth of between 0.1 and 0.5m at the down-wind end of each wetland.
From your reading, which voice do you think is most often used in your scientific discipline – active, or passive?
Present tense for diagrams and figures
If you use figures or diagrams to help explain what you did, refer to the figure or diagram using the present tense. Examples:
Table 1 above demonstrates the success of cloning in various animal species.
Figure 2 below shows methylation in mouse 2-cell embryos.
Past tense for results obtained
In the results section, use the past tense to detail the results you obtained. Examples:
Overall, more than 70% of the insects collected were non-phytophagous.
Results indicated that prolonged exposure to ultra-violet radiation had a positive correlation with the development of melanomas.
Following activation of NT oocytes with strontium, the cell cycle resumed in both groups.
Present tense to refer to figures, tables and graphs
As in the previous sections, use the present tense when you refer to figures, tables and graphs.
Examples: Figure 1 displays the comparative variation in the morphology of donor chromatin in both age groups of oocytes.
Table 1 below shows the stream flows calculated for each stream using Equation 1.
Present tense to explain significance of results
In your discussion section, you will explain the significance of the results. The present tense is normally used for this. Example:
Removal of vegetation for agricultural purposes appears to negatively affect the water quality of streams.
Past tense to summarise findings, with present tense to interpret results
Writers may use the past tense to summarise their findings, in combination with the present tense to explain or interpret what the results mean. Examples:
As the maxima and minima did not correspond to high and low tides, it is possible that the patterns observed may not be the result of mixing of waters with different concentrations.
Leaf carbon and phenolic content did not differ across sites, indicating that the response of secondary plant chemicals such as phenolics to water is complex.
In Example 1, the phrases ‘it is possible that’ and ‘may not be’ are used to indicate that other explanations are possible. This is an example of the use of limiting words to discuss findings in an academically tentative way.
Example 2 is less tentative. If you make a statement such as this, you are completely confident that your results and conclusion are correct.
A combination of tenses to highlight past research and future directions
In the final section of your thesis or report you summarise the main findings and the major implications of the study, point out any limitations, and offer suggestions for future research. To do these things you may use a combination of tenses. Example:
Although the study found evidence of tillage and irrigation within the study area, from the data collected it was not possible to determine if the effects of agriculture upstream cause (or caused) higher levels of total nitrogen downstream. Further studies are therefore necessary to determine the effects of agriculture on the health of Stringybark Creek.