It is vital to dispel the myth that dyslexia is a “category” of disorder.

The main symptom (in English) is poor reading accuracy or decoding skills.

We now know that the underlying weakness that causes poor decoding is difficulty with the sound structure of speech (phonological awareness).

This difficulty can vary from mild to severe, so dyslexia (like obesity) is properly viewed as a “dimension”. There is no absolute cut-off between children who are dyslexic and those who are not. Rather, children who start school with phonological difficulties are “at risk” of reading difficulties, but the risk is affected by factors such as a child’s other cognitive abilities, temperament, interests, and factors such as the environments in which they learn.

Research at York university and elsewhere shows that children who enter school with mild to moderate difficulties in phonological awareness respond well to targeted work such as Early Literacy Support. Such interventions are designed to promote awareness of speech sounds and include reading and spelling activities that link letters to phonemes. For children with more significant difficulties, a “wave 3” intervention is appropriate, such as “reading intervention with sound linkage”, as used in Cumbria.

We recently evaluated the effects of a modified version of this scheme in a trial in North Yorkshire. Year 1 pupils were taught daily for 20 weeks, alternating between individual and group sessions led by a trained teaching assistant. Group sessions included oral activities involving manipulation of the sounds of words, letter learning, and simple writing work.

One-to-one sessions comprised reading books at the easy and instructional levels.

The intervention was highly effective, with children making an average gain of one year within six months in reading (seven standard score points), and these gains were maintained after the intervention ceased. Importantly, despite these findings, 20 per cent of children at risk of dyslexia who receive interventions that are known to be effective show a poor response.

These children are typically those with severe phonological difficulties, sometimes accompanied by broader language impairments. They need targeted support that is intensive and continuous.

It is important that government recognises and provides for this group, who have lifelong difficulties with literacy. For many of them, mainstream provision is ineffective, and too often they are rationed to one course of intervention or left with in-class support. It is when dyslexia is severe and good provision is lacking that parents (including Ruth Kelly, the former education secretary) turn to specialist dyslexia schools in which their children can thrive in a whole-school environment that meets their needs.

Much greater understanding is required throughout the system so that fewer children have to wait until their reading difficulties become so marked that intervention is unlikely to succeed, or parents subscribe to alternative approaches that have no satisfactory evidence base.

Dyslexia is not a static condition that a person does or does not have. It is a continuous dimension associated with the risk but not the inevitability of literacy impairment. For most, early intervention is a cost-effective means of stemming the downward spiral of under-achievement and poor self-esteem that is too often associated with poor literacy. For some 20 per cent, more specialist help is needed. Indeed, there is merit in a staged approach to intervention but it must be emphasised that not all cases of dyslexia can be accommodated in mainstream education.

* www.york.ac.ukrescrl

Maggie Snowling is professor of psychology at the Univeristy of York