[senco-forum] Re: Dyslexia

Can I contribute this, from the NRDC Research Review of Developmental Dyslexia in Adults (Rice & Brooks 2004)

  "What is the prevalence of dyslexia?
  Although people sometimes speak of the ‘incidence’ of dyslexia, incidence is the frequency with which new cases occur and are diagnosed over a period of time (Barker et al., 1998). The proportion of dyslexic individuals in a population at any one time is properly called the ‘prevalence rate’ (Everitt & Wykes, 1999), although ‘prevalence’ is the term in general use.
  The previous section implies that any estimate of the prevalence of dyslexia will reflect the chosen definition and how it is operationalised. No two of those definitions could possibly identify the same individuals, or the same number of individuals, in any population. It has been shown that prevalence estimates for dyslexia are susceptible to definitional manipulation over a wide range (Snowling et al., 2000a). In the absence of a definition that provides unequivocal identification criteria, all statements about prevalence are guesses; they are value judgements, not scientific facts (Kavale & Forness, 2000). All the same, there can be no doubt that the higher the estimate is, the more likely it will be to confound dyslexia with ordinary reading difficulty.

      The British Dyslexia Association’s prevalence estimates of either 4 per cent for severe dyslexia or 10 per cent to include mild dyslexia have been described as ‘both theoretically and technically contentious’ (Pumfrey, 2001). Lower, but possibly no less contentious, are the estimate of 2.29 per cent for (what was then called) ‘specific reading retardation’ obtained from the Child Health and Education Study cohort (Rodgers, 1983) and the figure of 2.08 per cent given not as a prevalence estimate but as representing ‘the children in the cohort about whose dyslexia one can have the most confidence’ (Miles et al., 1993). In the original standardisation sample for the British Ability Scales (BAS), 8.5 per cent were defined as showing general reading backwardness, while a further 3.2 per cent were defined as showing ‘specific reading retardation’ (Tyler & Elliott, 1988). Because ‘specific reading retardation’ is a wider concept than developmental dyslexia, the prevalence of
 dyslexia must have been less than 3.2 per cent in the BAS standardisation sample.
  However, where dyslexics are identified by IQ-discrepancy methods, prevalence estimates may be particularly sensitive to teaching techniques; reading difficulty rates are higher with whole-language methods than with systematic and explicit phonics and highest of all among socially disadvantaged learners (Chapman et al., 2001; Nicholson, 1997). With all such estimates, unstable diagnoses raise further doubts about their reliability (Badian, 1999; Haslum, 1989).

  It may well be that in asking about prevalence we are posing an inappropriate question. If we take the perspective that ‘developmental disorders are not diseases that one does or does not have but are behaviorally defined dimensional traits along a continuum with fuzzy edges and a wide range of severity’ (Rapin, 2002), then questions about prevalence are misconceived; ‘there is no crisp partition between normalcy and disorder 
 even when there is no controversy regarding the identification of prototypic exemplars’ (Rapin, 2002)."

http://www.nrdc.org.uk/publications_details.asp?ID=11

Well worth reading.

Maggie

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