Combining possibly reciprocally dependent linguistic parameters in the quantitative assessment of aphasic speakers' grammatical output

Background: The grammatical output of aphasic speakers is often quantitatively analysed in order to establish an effect of treatment. In many methods of quantitative analysis both the number of utterances produced and (their) length are calculated. The difficulty is that these parameters may show a trade-off effect (i.e., may be inversely proportional to each other). That is, when the amount of information to be conveyed remains constant, an increase in the number of utterances may be compensated by a decrease in their length (and vice versa). As a result, a lack of increase—or even a decrease—in one of both outcome measures may be incorrectly interpreted as a lack of effect of treatment. Aims: The current study investigated whether combining the possibly reciprocal outcome measures percentage of utterances and (their) length into a single parameter increases the interpretability of the results obtained with a quantitative analysis of aphasic speakers' spontaneous speech. In the current study the procedur...