01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 33
Do ‘dyslexics’ and ‘non-dyslexics’ differ in kind or in degree?
The statistical evidence
It might be thought that all the analysis above points to the conclusion that dyslexics and nondyslexics
differ in degree rather than kind. The question whether ‘dyslexics’ differ in kind
(categorically) or in degree (dimensionally) from ‘non-dyslexics’ remains nevertheless one of
the most controversial in this field (Ellis, 1985; Fletcher et al., 1994; Fredman & Stevenson,
1988; Frith, 2001; Jorm et al., 1986b; Rutter, 1978; Share et al., 1987; Tyler & Elliott, 1988; van
der Wissel & Zegers, 1985). It may be a question that is best answered by reference to
complex models of reading development (e.g. Ellis & Large, 1987; Jackson & Coltheart, 2001;
and Stanovich, 1988).
It is important to specify whether the ‘non-dyslexics’ in the comparison group are good
readers or ‘ordinary’ poor readers. Much confusion appears to derive from failure to provide
this specification. The critical question in dyslexia research is not whether dyslexic people in
particular differ from ‘normal’ readers, neither is it how ‘poor’ readers in general differ from
“normal” readers. It is whether dyslexic people differ from other poor readers. Yet the
designs of many research studies leave this last question open. To test the hypotheses that
developmental dyslexics differ from both normal readers and ‘ordinary’ poor readers,
researchers need two control groups, not one. They also need to take both psychometric and
demographic measures, so that they can control for the effects of potentially confounding
variables. Without those controls, the effects of dyslexia are likely to be confounded with those
of conditions explicitly excluded from the concept of dyslexia (Chiappe et al., 2001).
In the Isle of Wight study, the presence of two categorically distinct traits in reading ability
was suggested by a bimodal distribution of reading ability—one with two peaks or humps, like
a Bactrian camel (Yule et al., 1974). However, while such a distribution might be interpreted
as evidence for a discrete phenomenon of specific reading difficulty it cannot be interpreted
as evidence for a genetically distinct syndrome of dyslexia, since methods of teaching as well
as biological differences could create a bimodal distribution (Rutter & Yule, 1975).
No evidence for a bimodal distribution in reading ability was found in the Child Health and
Education Study of the 1970 birth cohort (Rodgers, 1983), although the same data-set has
been analysed to investigate—but, as the investigators made clear, not to determine—the
prevalence of dyslexia (Miles et al., 1993). The Child Health and Education Study findings are
tentative with respect to both the lack of consensus for the criteria by which the dyslexic
cohort members were identified and also the uncertain sensitivity and specificity of the
chosen method of identification (Haslum, 1989).
By contrast, in the Connecticut study, a unimodal distribution—with a single peak or hump,
like a dromedary—suggested that reading ability is a single, dimensionally-distributed trait in
the population and that ‘dyslexics’ are those at the lower end of the continuum, not a discrete
group (Shaywitz et al., 1992). However, the logic of this interpretation has been questioned on
the ground that, although a dip in the observed distribution implies a mixture of underlying
processes, the converse does not necessarily hold. What is more, the failure to find a dip
could be due to both the obscuring effects of measurement error (Abelson, 1995) and the
presence of a few etiologically distinct factors (Pennington et al., 1992) one of which might be
rapid naming ability (Meyer et al., 1998; Wood & Grigorenko, 2001).