Active voice: Orophin measured the trees
Passive voice: The trees were measured by Orophin
Writing guides rarely say anything positive about the passive voice (positive things are rarely said about the passive voice by writing guides). But scientists carry on using it, as in:
“Seed source is considered to be important in forestry.”
or
“The survey instrument was developed in consultation with farmers’ representatives and local experts.”
But what is wrong with the passive voice?
Well, two things:
First, a phrase like “The survey instrument was developed in consultation with …” is vague, unless the authors make it clear elsewhere that they did the developing. Sometimes, the ambiguity may be deliberate; for example, some may feel more comfortable writing “it seems likely that incomes will continue to fall” than sticking their necks out and clearly stating “we think it likely that incomes will continue to fall.” Deliberate ambiguity can be useful in politics, but not normally in science.
Second, passive construction is more difficult for the brain to process: English speakers are used to the subject–verb–object sequence (‘I hit the ball’, rather than ‘the ball was hit by me’) (see Yellowlees Douglas’s book The Reader’s Brain).
In that case, why is the passive voice so common in scientific writing?
I suspect that one reason is scientists’ strong interest in what they are researching. These topics often correspond to the objects of active sentences (“Orophin measured the trees“). But when scientists write, what is foremost in their minds (the grammatical object) spills out first, and becomes the subject of a passive sentence. For example: “All trees, defined as woody plants (including bamboos and palms) of more than 10 cm diameter at 1.3 height, were measured in each of 53 farms.’ Such writing is indeed passive not only because it uses the passive voice, but because we are going with the flow of our thoughts, rather than thinking how best to express them.
But that doesn’t mean the passive should always be avoided
Nevertheless, injunctions to avoid the passive voice as if it were literary hemlock are unwarranted. The passive voice has a place in good scientific writing. For example, we may consciously want to stress or make a specific point about what would be the grammatical object of an active sentence. We can do this by making it the grammatical subject of a passive sentence “Focus groups, rather than individual interviews, were used because of time limitations” may work better than “Because of time limitations, we used focus groups, rather than individual interviews.”
The passive voice can also be appropriate in two standard components of scientific articles: the abstract and the methods section. Readers may scan abstracts without looking at the list of authors, in which case the impersonal feel of the passive seems natural. In methods sections, careless use of the passive may lead to ambiguity (see above), but in simple descriptions of procedures it can be more elegant than multiple repetitions of ‘I’ or ‘we’. In both cases, readers’ comprehension is aided, not hindered, because experience leads us to expect the passive in these sections.
In summary: use the passive voice for a specific reason, rather than just letting it creep in.