If the stress position is usually found at the end of a sentence, the topic position is usually found at the beginning, identifying the subject under discussion and providing context for all subsequent information. Consider two of Gopen and Shaw's examples: "Bees disperse pollen," and "Pollen is dispersed by bees." On the surface, both sentences seem to give us the exact same information. However, with "bees" in the topic position, we would expect to find the first sentence in a paragraph about bees. The second sentence, instead, tells a story about pollen, presumably leading the reader on to more information about pollen. Here, the topic position gives us the context for all following information.
The topic position also serves another important function: it links old information, previously presented in the text, to new information that will appear in the subsequent stress position. By properly exploiting topic and stress positions, the writer can present information seamlessly and logically, in a manner that is both easy to understand and pleasing to read; this is what professional writers refer to as the "flow" of a passage. However, improper use of both can lead to chaos in a seemingly well-constructed paragraph. Case in point: Gopen and Shaw's next example, taken from an actual published paper.
"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."
Here I'm going to quote Gopen and Shaw directly, because I can't express it any better than this:
"This is the kind of passage that in subtle ways can make readers feel badly about themselves. The individual sentences give the impression of being intelligently fashioned: They are not especially long or convoluted; their vocabulary is appropriately professional but not beyond the ken of educated general readers; and they are free of grammatical and dictional errors. On first reading, however, many of us arrive at the paragraph's end without a clear sense of where we have been or where we are going. When that happens, we tend to berate ourselves for not having paid close enough attention. In reality, the fault lies not with us, but with the author."
The main problem with this passage, they explain, is that the topic positions are constantly occupied by new material, in the place where readers expect to find context and linkage with old material. Because of this, it's difficult to determine exactly what story is being told: is it about the rate at which tectonic plates move? The recurrence time between shocks? The San Andreas fault? The topics jump around so much that one could read the passage through two or three times and not be entirely sure. Instead of finding the topics of the sentences in their expected positions at the beginning, as readers we must instead search through the passage and pick out the information that seems to be repeated relatively often, which we must then assume to be the topic of the entire paragraph. Many of the sentences seem to provide information about recurrence time, though this never explicitly appears in the topic position of any given sentence. If old material is not found in the topic position, it's also difficult to determine what, exactly, is the new information worthy of emphasis in each sentence. Using the assumption that the paragraph is about the recurrence interval between earthquakes, Gopen and Shaw proceed to highlight the most likely candidates for the available topic positions (linking backwards) and stress positions (important material presented for the first time), and they rewrite the passage as follows:
"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 roughly uniform. Therefore, nearly constant time intervals (at first approximation) would be expected between large ruptures of the same fault segment. [However?], the recurrence time may vary; the basic idea of periodic mainshocks may need to be modified if subsequent mainshocks have different amounts of slip across the fault. [Indeed?], the length and slip of great plate boundary ruptures often vary by a factor of 2. [For example?], the recurrence intervals along the southern segment of the San Andreas fault 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."
Although the paragraph is more slightly more readable, the logical gaps between sentences remain. To fill them, Gopen and Shaw propose "however", "indeed", and "for example", but only the original authors could tell us whether these connecting words represent their intended meaning. Even in the revised version, it is still unclear exactly what the authors are preparing us for in the rest of their article. Will we learn more about earthquake prediction? Or the difficulties in earthquake prediction? Something else entirely? Although the paragraph is structurally more sound after editing, its deficiencies in communication become much more obvious.
To improve the flow and readability of texts, then, Gopen and Shaw provide the following guideline:
"Put in the topic position the old information that links backward; put in the stress position the new information you want the reader to emphasize."
In this way, the writer is consistently providing context for readers, while drawing their attention to the material most deserving of emphasis. By using the topic position to link backwards, sentences flow seamlessly from one to the next, without logical gaps, while the stress positions contain the "take-home message" right where the readers expect to find it.
More to come! Stay tuned....
Gopen, G. and J. Shaw. "The Science of Scientific Writing." American Scientist. Nov.-Dec., 1990. Accessed from http://www.americanscientist.org/issues/pub/the-science-of-scientific-writing/.