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EBP Compendium: Summary of Systematic Review

Natural Sciences and Engineering Research Council of Canada
The Double-Deficit Hypothesis: A Comprehensive Analysis of the Evidence

Vukovic, R. K., & Siegel, L. S. (2006).
Journal of Learning Disabilities, 39(1), 25-47.

Indicators of Review Quality:

The review addresses a clearly focused question Yes
Criteria for inclusion of studies are provided Yes
Search strategy is described in sufficient detail for replication Yes
Included studies are assessed for study quality No
Quality assessments are reproducible No

Description: This is a systematic review of peer-reviewed studies examining the naming speed and phonological processing skills of individuals with developmental dyslexia in order to investigate the double-deficit hypothesis theory.

Question(s) Addressed:

(The following questions were discussed in the article. Each was used as a topic heading in the results section.)

  1. Is dyslexia characterized by naming speed difficulties?
  2. What is naming speed's contribution of independent variance to reading ability?
  3. Should naming speed by characterized independently from phonological awareness?
  4. What is the nature of the relationship between naming speed and reading development?
  5. What does the extant research conclude about the three subtypes?
  6. Who is slow on RAN?
  7. What is the efficacy of interventions for dyslexia?

Population: Individuals with dyslexia or reading difficulties

Intervention/Assessment: Studies designed to test or investigate the double-deficit hypothesis of developmental dyslexia (In the double-deficit hypothesis, both naming speed deficits and phonological skill deficits exist in an individual)

Number of Studies Included: 36

Years Included: Not stated



  • Assessment/Diagnosis
    • Assessment Areas
      • Reading
        • The evidence for the double-deficit hypothesis suggesting that reading impairment is a result of independent deficits in naming speed and phonological processing remains inconclusive.
        • Overall, evidence from the included studies suggest that naming speed deficits are characteristic of a subset of individuals with dyslexia, generally the more severe dyslexic individuals. "[H]owever, the question of whether naming speed deficits characterize dyslexia is largly unresolved" (p. 35).
  • Treatment
    • Language
      • Reading - "The hypothesis that children with a naming speed deficit would not benefit from phonlogical interventions in not supported" (p. 45).

Keywords: Literacy, Language Disorders, Written Language Disorders

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Added to Compendium: March 2012

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