01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 41
dyslexia, we cannot operationalise the concept? Does it mean that, although we can
operationalise the concept of dyslexia, we can do so only in an arbitrary manner? Or does it
mean that we can operationalise some concepts of dyslexia but not all of them and that
feasibility of operationalisation necessitates both a broadening of the concept and a
corresponding reduction in its unique explanatory potential?
As is clear from Appendices 1 and 2, there are many definitions of dyslexia and the
differences between them are striking. In the stipulative definition that we have adopted in
this review, the guiding concept is widely if not universally agreed: dyslexia is ‘a
neurodevelopmental disorder with a biological origin, which impacts on speech processing
with a variety of clinical manifestations’ (Frith, 1999). However, we cannot prove a biological
origin in any individual case. We must take care not to confuse correlation with causation; at
best, we can think only in terms of probability. We have to establish the exact degree of
probability by reference to a base-rate for the prevalence of dyslexia. To determine a baserate,
we need to operationalise the concept. As yet, there is no agreement as to how this
should be done. That is the crux of the problem.
A diagnosis of dyslexia is a theory, but the diagnostician cannot estimate the likelihood
of its being correct.