There are several questions about tense use in paper writing. My question is specific to the results section. What tense should it be used when reporting the results of the paper itself?
"Two groups out of three had a higher incidence of..."
"Group A is taller than group B"
I guess both might work but I can't find any good rule to follow.
There is one important difference between
Group A is taller than group B.
and
Group A was taller than group B.
When you write in the present tense, you claim that your finding is always true, in the sense of an eternal truth. There is nothing wrong with reporting results in the present tense if that is what you want to express. However, if some other paper reports results that contradict your findings such a general statement quickly becomes wrong.
So scientist usually are careful with what they say, and a statement in the past tense, if it reflects your observations, will always be true in the framework of your paper. Conflicting findings cannot possibly invalidate your statement in any way, because you simply reported how it actually was.
So there is basically no rule that you follow when writing in past tense, it is a decision you take about what you want to express.
The results section, as the name itself suggests, 'reports' the findings. Reporting should take place in the past tense in a passive voice. That would be the first sentence you quote.
Additional thoughts: While the past tense clause is difficult to argue against, some may argue that active voice is easier to read and perhaps should be preferred. It may boil down to preference, and you may have your own, as long as you stay consistent within the section/article. Make sure it does not conflict with reporting, see the other answer on that.
Contrary to other answers here, I see nothing wrong with reporting results in the present tense. When you read old newspapers from a century or two ago, and they write in the present tense, you don't take that to mean that the things in them are still true, and you don't consider it an error on the part of the writer if something they assert is no longer true --- "Even with the arrival of the Benz Patent-Motorwagen, the horse and buggy is still the favoured method of transfort for society ladies".
Any sensible reader of an academic paper understands that the assertions made relate to research conducted at that time and that assertions about the state of the world related to the time and place under consideration in the research. In particular, when you refer to groups of research participants, it is understood that you are referring to a particular group of people as they existed at the time of the research. (If this were not true, how would we ever report age data for such groups?)
I disagree with the other answer here asserting that writing in the present tense makes a claim that something is "eternally true". That is simply ridiculous --- people make assertions in the present tense all the time, and they rarely intend these assertions to contain eternal truths. To the contrary, any sensible reader will interpret writing in the present tense as meaning that the relevant facts are asserted to be correct at the time of the research (i.e., usually about a year or two prior to publication).
In terms of which tense is better, that is contextual, and you will need to use your best judgment as to what sounds clearer and more accurate. Either tense should be legitimate so long as it is clear to the reader (often from context) when the findings were made. Unless there is a good reason to the contrary, present tense will usually be interpreted as referring to the time at which the research was conducted. Past tense is a bit trickier --- depending on context, it might be interpreted in this same way, or it might be interpreted as meaning that the asserted fact was true at some time prior to the research being conducted. If you are using past tense, you should be careful to ensure that you are not implicitly suggesting a contemporaneous change in facts occuring at the time of the research --- e.g., saying "Group A was taller than group B" might suggest that they are no longer taller now.
The results of a paper aren't really observations about how the world is: they are the results of measurements made, or analyses done. I think it's more natural to report these in the past tense, because even when you are writing the paper, the analyses were done in the (hopefully recent) past.
For example, you wouldn't really write
Group A is taller than Group B.
You would write something like
The average height in Group A was 3.7cm [2 SD] greater than the average height in Group B.
I want to write "was" here, because it could be replaced with "was found to be" or "was calculated to be". It is not wrong to say "is", but it feels unnatural to me.
There are some situations where "is" feels more natural - when you're really reporting on something out there which doesn't require any new analyses. For example:
In Smith et al.'s data set, every member of Group A is taller than every member of Group B, which is a striking difference.
The data set is fixed in time and eternal; a "was" here would imply that the data set was corrected later. But we'd write
Smith et al. found a statistically significant difference between the heights of Group A and Group B
because they found this difference in the past.
Both are common. The present tense is more lively and it has one other advantage: It allows you to separate what you did from what previous authors have done. "The chemical reactions are divided into three groups, whereas Adam et al. (2021) divided them into two groups."
Unless an event definitely occurred in the past, such as "the comet impacted Jupiter", I always use the present tense: "The results show ...", "The values are calculated with ...". The choice of tense should be uniform; unnecessarily switching between tenses would be confusing.
In our experiment, group A is taller than group B.But then most of my reporting is more mathematical and more likely to be true for all time.