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<Paper uid="P89-1011">
  <Title>Table I</Title>
  <Section position="6" start_page="87" end_page="88" type="evalu">
    <SectionTitle>
RESULTS
</SectionTitle>
    <Paragraph position="0"> Table 1 below gives a selection of the more important results for each strategy by transcription scheme for the test umtence in (2). Column 1 shows the total number of access paths initiated for the test sentence under each strategy. Columns 2 to 6 shows the number of words in all the cohorts produced by the particular access strategy for the test sentence after 2 to 6 phonemes/segments of the transcription have been incorporated into each access path. Column 7 shows the total number of words which achieve a complete match during the application of the particular access strategy to the test sentence.</Paragraph>
    <Paragraph position="1"> Table 1 provides m index of the efficiency of each access strategy in terms of the overall number of candidate words which appear in cohorts and also the overall number of words which receive a full match for the test sentence. In addition, the relative performance of each strategy as the ~ption scheme becomes less determinate is clear.</Paragraph>
    <Paragraph position="2"> The test sentence contains 12 words, 20 syllables, end 45 phonemes; for the purposes of this experiment the word a in the test sentence does not trigger a look-up attempt with the word strategy because cohort sizes were only recorded for sequences of two or more phonemes/segments. Assuming a fine class trmls=iption serving as lxe-lexical input, the phoneme strategy produces 41 full matches as compared to 20 for the strong syllable strategy. This demonstrates that the strong syllable strategy is more effective at ruling out spurious word candidates for the test sentence. Furthermore, the total number of candidates considered using the phoneme strategy is 1544 (after 2 phonemes/segments) but only 720 for the strong syllable strategy, again indicafng the greater effectiveness of the lanef strategy. When we  consider the less determinate tran.scriptlons it becomes even clearer that only the strung syllable slrategy remains reasonably effective and does not result in a ma~ive increase in the rmmber of spurious candidates accessed and fully matched. (The phonmne strategy resets are not reporud for mid end broad class tramcrlptlons because the cohort sizes were too large for the database query facilities to cope reliably.) The word candidates recovered using the phoneme strategy with a fine class transcription include 10 full matches resulting from accesses triggered at non-syllabic boundaries; for example arraign is found using the second phoneme of the and rain. This problem becomes considerably worse when moving to a less determinate transcription, illustrating very clearly the undesirable consequences of ignoring the basic linguistio constraint that word boundaries occur at syllable boundaries.</Paragraph>
    <Paragraph position="3"> Systems such as TRACE (McClelland &amp; Elman. 1986) which use this strategy appear to compensate by using a global best-fit evaluation metric for the entire utterance which s~rongly disfavours 'unattached' input. However.</Paragraph>
    <Paragraph position="4"> these models still make the implausible claim that candid~_!e~ llke arraign will be highly-activated by the speech input.</Paragraph>
    <Paragraph position="5"> The results concerning the word based strategy presume that it is possible to determinately recognise the endpuint of the preceding word. This essmnption is based on the Cohort theory claim (e.g. Marslan-Wilsun &amp; Welsh, 1978) that words can be recogulsed before their acoustic offset, using syntactic and semantic expectations to filter the cohort. This claim has been challenged experimentally by Grosjean (1985) and Bard et al. (1988) who demcmstrate that many monosyllabic words in context are not recognised until after their acoustic offset. The experiment reported here supports this expesimental result because even with the fine class transcription there are 5 word candM~t_~ which extend beyond the correct word boundary end 11 full matches which end before the correct boundary. With the mid clam tran.un'iption, ~e~ numbers rise to 849 end 57.</Paragraph>
    <Paragraph position="6"> respectively. It seems implausible that expectation-based corm~ainm could be powerful enough to correcdy select a unique candidate before its acoustic offset in all contexts. Therefore, the results for the word strategy reported here are overly-optim.isdc, because in order to guarantee that the correct sequence of words are in the cohorts recovered from the input, a lexical access system based on the word strategy would need to operate nondemrministically; that is, it would need to consider several pumndal word boundaries in most cases.</Paragraph>
    <Paragraph position="7"> Therefore, the results for a practicM syr.em based on Otis approach am likely to be significantly worse.</Paragraph>
    <Paragraph position="8"> The syllable strategy is effective under the assumption of * determinate and accurate phonemic prelexieal representation, but once we abandon this idealisation, the effectiveness of this strategy declines ~trply. Under the plaus~le assumption that the pre-lexical input reprmemation is likely to be least accurate/deanminate for tmslressed/weak syllables, the sw~ng syllable strategy is far more robust. This is a direct consequence of triggering look-up attempts off the more determinate parts of the pre-lexical representation. Further theoretical evidence in support of the strong syllable strategy is provided by Cutler &amp; Carter (1987) who demmmtrate that a listener is six times more likely to e~mter a word with a prosodically strong initial syllable than one with a weak initial syllable when listening to English speech. Experimental evidence is provided by Cutler &amp; Norris (1988) who report results which suggest that listeners tend to treat strong, but not weak, syllables as appropriate points at which to undertake pre-lexical segmentation of the speech input.</Paragraph>
    <Paragraph position="9"> The architecture of a lexical access system based on the syllable strategy can be quite simple in terms of the organisation of the lexicon and its access routines. It is only n~essary to index the lexicon by syllable types (Church, 1987). By contrast, the strong syllable strategy requires a separate closed.class word lexicon end access system, indexing of the open-class vocabulary by strong syllable and a more complex matching procedure capable of inhering preceding weak syllables for words such as d/v/s/on. Nevertheless, the experimental results reported here suggest that the extra complexity is warranted because the resulting system will be considerably more robust in the face of inacct~rate or indeterminate input concerning the nature of the weak syllables in the input utterance.</Paragraph>
  </Section>
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