Results for reading times resembled those for reading rates with the exception, again, o f the Sign length x Group interaction. This was absent in analyses o f reading rate. Like the previous experiment, this might be due to the manipulation o f item length, such that the difference in rate between long and short signs (for the Auslan group) was less than that for long and short words (for the Oral subjects). This was confirmed: the difference in primary linguistic rate was smaller for Auslan subjects than Oral subjects [F (l, 38) = 4.12; p < .05], though the SE group differed from neither.
To assess the effect o f presentation condition on performance, the production rates obtained in Experiments 2 and 3 were compared. The Auslan group was slower with orthographic presentation [F (l, 19) = 63.38; p < .0001] as was the SE group [F(l, 19) = 39.66; p < .0001], although the Oral group achieved comparable rates in both experiments. This paralleled the memory span results exactly.
In summary, the completion times and rates o f the three communication groups were significantly affected by the item length in their communication mode, therefore these stimuli could be used to test for an item length effect. The absence o f a Sign length x Group interaction with the reading rate data suggested that within-group analyses would be a suitable way to test for the presence o f a sign length effect in this experiment. The Oral group performed significantly faster than the remaining subjects, who were also slower with orthographic presentation. This suggested that the two signing groups may
have been poorer readers. Finally, the time difference between short and long signs for Auslan subjects was less than for words for the Oral group, hence the relatively large English length effect may have been due to a stronger manipulation o f item length for the latter group.
5.4.5 Discussion
The major result o f this experiment was that item length effects in the preferred communication mode were obtained for all groups, including the Auslan subjects. The analyses also suggest that a real sign length effect did indeed underlie the marginal result for the signing subjects in Experiment 2. This was supported by the finding that the manipulation o f item length produced smaller effects on repetition and reading rates for this group, which would explain the smaller impact o f production rate on memory span for them.
Additionally, the SE group's results tended to fall between the Auslan and Oral group on both speed and span measures. Thus the results confirm the importance o f item length as a determinant o f memory span irrespective o f whether groups are considered individually or when their collective matrix o f results is examined.
Speed differences between lists were upheld as predicted, indicating that the items could be used to test the effects o f rehearsal rate on memory. The absence o f a Sign length x Group interaction in the production rate analyses implies that a within- group analysis o f memory performance would be an appropriate way to test for sign length effects.
The relation between memory span and reading rate is plotted in Figures 5.6a and 5.6b, where the dotted lines indicate the lines o f best fit for the three groups. (These lines were fitted to group means by the method o f least squares). Two features are noteworthy. The first concerns within-group characteristics. Both the individual scores (in Figure 5.6a) and the group means for each list (in Figure 5.6b) tend to fall on the one straight line for each o f the Auslan, SE and Oral groups. This suggests that speed is exerting a constant influence over each group's performance. Secondly, there is a close correspondence between the three speed-span functions, for the performance o f all the groups follows a virtually parallel slope and would have similar intercepts. This suggests that a single factor - rehearsal speed - might have determined memory performance, irrespective o f the mode o f the items which were rehearsed.
The finding that the signing groups achieved higher recall levels with signed presentation, fits with the conclusion reached in Experiment 1 that their memory code is a sign-based one.
Reading skill may also have contributed to performance. Treiman and Hirsh- Pasek (1983) suggest that a non-primary linguistic presentation condition may impair performance either because there is a lack o f organisational structure for the code being used or because there is a translation problem between the primary and secondary codes. This applies particularly to the SE and Auslan subjects, whose memory spans were shorter and production rates lower under orthographic than primary linguistic presentation, and who commented that the long English stimuli were "hard words".
A A f Ä A A <MXSO ■ A « A '< « K O » Oft H O I O A A A O A > A m « o o CD M < 2 D A Ö Ö » Ö X *B « iC * ö ■ A I O I A O « I Ä A / Ö ■ & 0 O < A » ° Auslan a SE ■ Oral Reading rate