01 NRDC Dyslexia 1-88 update - Texthelp

34
Research Report
The statistical findings are mixed and do not present irrefutable evidence for differences in
either kind or degree between dyslexic and non-dyslexic people.
The psychometric evidence
Although ‘dyslexics’ are easily shown to differ from good (or even adequate) readers on a
number of reading-related assessments, we need to ask whether the same assessments
differentiate ‘dyslexic’ from ‘non-dyslexic’ (i.e. ordinary) poor readers (Stanovich, 1988). It now
seems that a number of reading-related characteristics once thought to be specific to
dyslexia are shared with other novice readers (Ellis, 1985; Taylor et al., 1979). These
characteristics include reversal errors with letters of the alphabet (Fowler et al., 1977; Mann
& Brady, 1988; Worthy & Viise, 1996), directional sequencing errors (Vellutino, 1979),
pseudoword reading difficulties (Bishop, 2001), phonemic segmentation difficulties (Cole &
Sprenger-Charolles, 1999; Metsala, 1999) and spelling of sight vocabulary (Scarborough,
1984). The characteristics are also shared with neurologically normal people who have had no
opportunity to learn how to read (Castro-Caldas et al., 1998; Kolinsky et al., 1994; Lukatela et
al., 1995; Morais et al., 1979). Moreover, between-group differences on those measures are
also associated with differences between phonics-based and whole-word methods of teaching
reading (Alegria et al., 1982; Thompson & Johnston, 2000). Students tend to perceive words in
the way that they are taught to perceive them (Huey, 1908). This appears to be the case
whether or not they are taught in a transparent orthography (Cardoso-Martins, 2001), where
each phoneme is always or almost always represented by the same grapheme, as in Finnish
(always) or Italian (almost always).
Although dyslexic people are believed by some scholars to differ dimensionally rather than
categorically from non-dyslexics (Shaywitz et al., 1992), this may not mean that the most
impaired readers are dyslexic (see also page 61). When IQ-discrepancy criteria have been
used to distinguish dyslexics from ordinary poor readers, the ordinary poor readers have
obtained lower scores than dyslexics on memory span, segmentation, and rhyme tasks
(Fawcett et al., 2001) and also on a pseudoword repetition task (Jorm et al., 1986a).
With respect to differences unrelated to reading, a number of apparently interesting
associations between ‘dyslexia’ and other variables (such as blue-collar social status,
inconsistent left-handedness and autoimmune disorders such as hay fever) have been found
to disappear when subject to the scrutiny of multivariate techniques of data analysis (Haslum,
1989). This finding is an example of a general warning seldom heeded in folk psychology:
namely, that simple bivariate correlations do not establish a causal relationship and cannot
determine a causal direction. A more specific warning, against reification, is also appropriate:
‘Categorical diagnoses do not refer to real discrete entities; they are only meaningful as
approximate descriptions that remind the clinician of prominent characteristics of specific
combinations of quantitative traits’ (Cloninger, 2000).
Psychometric findings suggest that differences between dyslexic and non-dyslexic readers
are differences in degree, not differences in kind.
Are diagnoses of dyslexia stable across methods of ascertainment?
A sample of dyslexic people defined by IQ-discrepancy using a cut-off of one standard
deviation (1 SD) below the mean on a continuous measure of decoding skill will be about