The Delphi Study: Implications of A New Definition of Dyslexia

This article was updated to reflect the amendments to the articles (Dr Anna Smith, 03/2026).

Professor Julia Carroll, Caroline Holden and Paul Thompson have released a paper looking at a new definition of dyslexia. 

The highly anticipated Delphi study was published in February 2025 and presented at the SpLD Assessment Standards Committee SASC conference. It outlines a new definition for dyslexia and will have implications for the assessment of specific learning difficulties (SpLD). The papers are free to access but are yet to be peer-reviewed and further edits may occur.

We highly recommend that dyslexia assessors and teachers read the articles in full, these are available on the Open Science Framework and also on the SASC website.

You can read more about the definition in the article below.

What is the new Delphi definition of dyslexia?

The core message from the panel is that dyslexia is primarily a difficulty with reading and spelling, rooted in underlying processing issues.
  
The new definition is as follows;

• Dyslexia is a set of processing difficulties that affect the acquisition of reading and spelling
• In dyslexia, some or all aspects of literacy attainment are weak in relation to age, standard teaching and instruction, and level of other attainments
• Across all languages, difficulties in reading fluency and spelling are key markers of dyslexia
• Dyslexic difficulties exist on a continuum and can be experienced to various degrees of severity
• The nature and developmental trajectory of dyslexia depends on multiple genetic and environmental influences
• Dyslexia can affect the acquisition of other skills, such as mathematics, reading comprehension or learning another language
• The most commonly observed cognitive impairment in dyslexia is a difficulty in phonological processing (i.e., in phonological awareness, phonological processing speed or phonological memory).
  However, phonological difficulties do not fully explain the variability that is observed
• Working memory, processing speed and orthographic skills can contribute to the impact of dyslexia
• Dyslexia frequently co-occurs with one or more other developmental difficulties, including developmental language disorder, dyscalculia, ADHD, and developmental coordination disorder 

A closer look at how the Delphi definition differs from the Rose definition

While both definitions place importance on phonological processes as key cognitive markers when diagnosing dyslexia, there are some key differences. A useful summary of the main differences can be found on the SASC website. Below are some of our own observations;

1. Focus on fluency

Note the focus on fluency. This is a key factor when testing older students and adults, supporting the notion that dyslexia can be present even when literacy attainment (particularly in terms of accuracy) is within the average range. It also acknowledges that fluency can be important when assessing dyslexia in languages other than English.

The matter of discrepancy is addressed directly with the statement that a notable difference between attainment and ability can serve as a ‘useful indicator of a specific learning difficulty but is not sufficient for a diagnosis in and of itself’. This suggests that ability tests will continue to form some part of assessment.  

2. Multifactorial etiology

Dyslexia is seen as multifactorial in its etiology and this draws our attention to the importance of background information concerning heritability as well as environment. 

3. Orthographic processing 

This is a term that is introduced but the evidence is not as strong for its causal role in dyslexia as it is for phonological processing. The authors propose that ‘orthographic processing difficulties are best regarded as a useful marker for dyslexia, and that further research investigates the role of orthographic processing’. There is a free webinar available online with Nancy Mather (co-author of the ToD) on orthographic knowledge which may be of interest to assessors.

4. Inclusion of adults

A key advantage of this new definition, however, is that it makes reference to adults as well as children, by stating that, ‘while some older children and adults with dyslexia continue to experience word-level reading problems, others mainly have difficulties in reading and writing fluency, and in spelling’. This is an important point for dyslexia practitioners who work with older students and adults. 

What will it mean for a dyslexia diagnosis?

Anyone diagnosed under the previous (Rose) definition of dyslexia will continue to meet the criteria for diagnosis – there is no suggestion to the contrary.

What are the current implications for dyslexia assessors and practitioners?

SASC has recommended that practitioners make use of the new definition. 

What definition should I be using in my current assessment reports?

If you are planning to renew your APC, the Delphi definition should be used. 

How was the study carried out?

The study was conducted with a panel of dyslexia experts including academics from several leading UK universities, including Warwick, Oxford, and Coventry, specialist teachers, educational psychologists, and individuals with dyslexia. The authors asked them for their views on a set of key statements about dyslexia. They carried out two survey rounds, in each case accepting statements with greater than 80% consensus and reviewing and revising other statements using feedback from the expert panel. This was followed by a discussion with a subset of the panel around a few statements with marginal consensus.