This post continues a series on principles of scientific writing, as proposed by Gopen and Swan (1990). We've already covered the first three principles (to understand the material in this post, you need to read the previous material starting here).
Readers expect a unit of discourse (a sentence) to be a story about whoever or whatever is mentioned first, i.e., in the topic position. The example I've been using is "squirrels hide acorns" versus "acorns are hidden by squirrels". The first version suggests the focus is on squirrels, whereas the second one indicates the focus is on oak trees and/or seed dispersal. These examples also illustrate active versus passive voice.
The importance of the topic position forms the third principle of scientific writing:
Place the person or thing whose "story" a sentence is telling at the beginning of the sentence, in the topic position.
Another expectation for material in the topic position is that it provides a linkage backward (to previous information). Previously introduced material is called "old information", and its placement in the topic position helps readers follow the author's logic. Now we consider the fourth principle, which can be stated this way:
Place appropriate "old information" (material already stated in the discourse) in the topic position for linkage backward and put in the stress position the information you want the reader to emphasize (for contextualization forward).
Have we done this with our example? In the abstract below, I've highlighted in pink the "old information" that should be in the topic position. New information (yellow) should be in the "stress position" for contextualization forward.
The discrete-dipole approximation (DDA) is [often] used in scattering calculations, but its accuracy is unclear in relation to that of other computational methods such as complex-conjugate gradient algorithms and fast-Fourier-transform methods. The accuracy of the DDA was tested in computations of scattering and absorption by different targets: isolated, homogeneous spheres and two contiguous spheres. For dielectric materials (¦m¦ ≲ 2), the DDA permitted calculations that were accurate to within a few percent.
We've done a pretty good job of placing the old information in the topic position for linkage backward and new information in the stress position. The only quibble might be with the phrase "For dielectric materials". This term is suddenly introduced without explanation, but presumably would be understandable to experts in this field.
To illustrate this principle further, I will use the example given in Gopen and Swan:
Large earthquakes along a given fault segment do not occur at random intervals because it takes time to accumulate the strain energy for the rupture. The rates at which tectonic plates move and accumulate strain at their boundaries are approximately uniform. Therefore, in first approximation, one may expect that large ruptures of the same fault segment will occur at approximately constant time intervals. If subsequent main shocks have different amounts of slip across the fault, then the recurrence time may vary, and the basic idea of periodic mainshocks must be modified. For great plate boundary ruptures the length and slip often vary by a factor of 2. Along the southern segment of the San Andreas fault the recurrence interval is 145 years with variations of several decades. The smaller the standard deviation of the average recurrence interval, the more specific could be the long term prediction of a future mainshock.
At first read, one has the impression that this is a fairly straightforward description of earthquakes and tectonic plates and is written well. However, by the time we reach the end of the passage we are feeling somewhat befuddled and would be hard-pressed to say exactly what the point of this paragraph was. Gopen and Swan analyze this example and conclude that the main problem is that virtually every piece of new information makes its first appearance in the spot we expect to find the old, familiar information.
If your writing continually begins sentences with new information and ends with the old information, you will definitely disorient your readers and reduce comprehension, as the above example does. So how would one go about fixing the above paragraph? Take a second look at the fourth principle stated above and see if you can rewrite the paragraph.
I'll provide the revision suggested by Gopen and Swan in the next post along with some additional explanations.