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Eliminating bias in selection
criteria reveals equal prevalence
of developmental surface and
phonological dyslexia.
Dean P. Wybrow & J. Richard Hanley
3rd July 2013
Developmental surface dyslexia?

Marshall (1984) proposed that the dual route model could be applied to
developmental dyslexia and one should expect to find both phonological
and surface developmental dyslexia.

Developmental phonological dyslexia is well established in theory
(Campbell & Butterworth, 1985, Rack et al., 1992, Snowling, 2000) .

Stage Models predict developmental surface dyslexia (Frith, 1985)

Some case studies identified individual cases of developmental surface
dyslexia (e.g.Coltheart et al., 1983; Hanley & Gard, 1995)

Case studies do not reveal how prevalent surface dyslexia is, so this
question needed to be addressed.
Subtyping studies
Castles and Coltheart (1993): Varieties of Dyslexia

56 eight to fourteen year-old dyslexics (all at least 18 months behind in their
reading)

56 control participants with the same mean age

Reading tests: irregular words (IWR) and nonwords (NWR)



They were looking for:
A) specific impairment in irregular word reading
B) specific impairment in nonword reading
Subtyping studies

PURE dyslexia

When age is on the x axis, you can identify PURE surface or phonological dyslexia.
– They are impaired in irregular word reading or nonword reading but not the other.
10/56 dyslexics who fell below the
confidence intervals for IWR but not for NWR
8/56 dyslexics who fell below the confidence
intervals for NWR but not IWR
consistent with a surface dyslexic profile
consistent with a phonological dyslexic profile
Subtyping studies

RELATIVE dyslexia

When irregular word reading or nonword reading are on the x axis, cases falling outside the C.I.s are
RELATIVE surface dyslexia or phonological dyslexia.
– They are impaired in both, but worse on one task than the other.
16/56 dyslexics who fell below the
confidence intervals for IWR but not for NWR
29/56 dyslexics who fell below the
confidence intervals for NWR but not IWR
(10 of these were pure)
(8 of these were pure)
Subtyping studies

85% of the dyslexics showed some kind of dissociation

16 were poor at irregular word reading relative to nonword reading.
 10 of these were pure surface dyslexia

29 were poor at nonword reading relative to irregular word reading.
 8 of these were pure phonological dyslexia

Vindication of the application of the dual route model to developmental
processes as well as adult reading

Dissociative cases are the norm rather than the exception
Problem with the study

The control participants in this study were matched to the
dyslexics on chronological age

According to Snowling, Bryant & Hulme (1996), any differences
could be a consequence of difference in reading ability rather than
a cause
 control and dyslexic participants should be matched on reading
level

Only if surface dyslexics perform worse on IWR than reading level
controls are they genuinely surface dyslexic

Therefore the number of surface dyslexics may be artificially
inflated.
Stanovich et al (1997)

Following comments by Snowling et al., Stanovich et al. sought to revise
the method.

A reading level match was used alongside a chronological age match.

Controls matched to the dyslexics on WRAT-R reading subtest
Stanovich et al (1997)
• IWR regressed onto NWR for controls with dyslexics superimposed
Y axis: exception words = irregular words
CA match: 15 relative surface dyslexics
(Ys on graph)
No controls shown on this graph
RL match: 1 relative surface dyslexic
(dyslexics are Xs on graph)
Controls are squares
Stanovich et al (1997)
• IWR regressed onto NWR for controls with dyslexics superimposed
CA match: 15 ‘relative surface dyslexics’
dyslexic’
(Ys on graph)
No controls shown on this graph
RL match: 1 ‘relative surface
(dyslexics are Xs on graph)
Controls are squares
Stanovich et al (1997)
• IWR regressed onto NWR for controls with dyslexics superimposed
CA match: 15 ‘relative surface dyslexics’
dyslexic’
(Ys on graph)
No controls shown on this graph
RL match: 1 ‘relative surface
(dyslexics are Xs on graph)
Controls are squares
Stanovich et al (1997)

When using chronological age (CA) match, 15 ‘relative surface
dyslexics’ were identified

When using reading level (RL) match, only one ‘relative surface
dyslexic’ was found.
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Similar results found by Manis et al. (1996)

Neither found many pure cases, but rather found many relative cases,
which begs the question as to whether these relative cases are like
control children.

Stanovich et al. claimed that the lack of surface dyslexics in RL match is
because these discrepancies are expected at that level of reading ability the Delay Hypothesis.
Problems with the RL match
1 (Jackson & Coltheart, 2001)

Good IWR performance depends on a large sight vocabulary.
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This is built up over time.
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The dyslexics are older than the controls, and therefore have had
more time to develop their sight vocabularies.
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It would therefore be a lot to expect a surface dyslexic’s IWR
performance to be worse than that of a younger control.
Problems with the RL match
2
(Douklias, Masterson & Hanley, 2010)

Stanovich et al. controls were matched using the WRAT-R;
Manis et al. controls were matched with Woodcock Word Identification.
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a)
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Both of these contain large numbers of irregular words, e.g. TERPSICHOREAN
and PSEUDONYM.
 Limiting difference between dyslexics and controls in IWR before you even start
your search.
 Therefore finding none who are deficient in IWR – no surface dyslexics.
b)

They do not contain any nonwords.
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Therefore no limitation on finding phonological dyslexics.
Current Study - Issues

CA match is not appropriate, and neither is the RL, so what is an
appropriate comparison group?

In order to see whether the kinds of surface dyslexia-like discrepancies
found thus far can be found in the normal population:
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Match on the independent variable

Match control group to dyslexics on nonword reading to find surface
profiles (NW match).

Match control group to dyslexics on irregular word reading to find
phonological profiles (IW match).
Current Study - Issues

Chronological age match should reveal large numbers of
dyslexics with a discrepancy between irregular word reading
and nonword reading.

Reading level match should reveal fewer of these cases, but
a stable number of phonological profiles.

When matching on NWR, will we find surface dyslexialike discrepancies between IWR and NWR in the control
population?
Current Study - Method


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WRAT-3 reading subtest
21 irregular words (IWR) (e.g. YACHT)
30 nonwords (NWR) (e.g. POLD)

Both taken from Stanovich et al. (1997)
Current Study Part One - Replication

41 dyslexic children (identified through SEN)
(DYS group)

43 children with same mean age as dyslexic
group (CA group)

61 children with same reading level as dyslexic
group (RL group)
Results – CA match, poor at IWR
43 controls matched to dyslexics
on chronological age
R= 0.58, F(42) = 21.2
(p<.001)
Individual 90%
confidence intervals
Falling below the lower
line reveals poor IWR
relative to NWR.
19/41 dyslexics poor at
IWR relative to NWR
Results – CA match, poor at IWR
43 controls matched to dyslexics
on chronological age
R= 0.58, F(42) = 21.2
(p<.001)
Individual 90%
confidence intervals
Falling below the lower
line reveals IWR
relative to NWR.
19/41 children poor at
IWR relative to NWR
Results – RL match, poor at IWR
61 younger controls matched to
dyslexics on reading ability
R= 0.79, F(60) = 102
(p<.001)
Individual 90%
confidence intervals
Falling below the lower
line reveals IWR
relative to NWR.
3/41 children poor at
IWR relative to NWR.
Current Study Part One - Summary

Results from Castles & Coltheart (1993) and
Stanovich et al. (1997) replicated
 CA match gives many dissociative cases
 RL match gives many phonological cases (11) but
just three surface cases.

and so onto the new method...
Current Study Part Two

41 dyslexic children (identified through SEN)
(DYS group)

38 children with same mean NWR as dyslexic
group (NW group).
Results – NW match, poor at IWR
48 controls matched to dyslexics
on nonword reading
R= 0.82, F(47) = 103
(p<.001)
Individual 90%
confidence intervals
Falling below the lower
line reveals IWR
relative to NWR.
5/41 poor at IWR
relative to NWR.
Results – are these pure surface cases?
Participant
IWR deficit
NWR deficit
Dissociation
Slow reading
IWR
Slow reading
NWR
PURE?
7
YES**
NO
CLASSICAL
NO
NO
YES
9
YES**
NO
CLASSICAL
YES**
YES**
YES
11
YES**
YES**
STRONG
YES**
YES**
NO
12
YES**
YES**
STRONG
YES**
YES**
NO
22
YES**
YES*
STRONG
NO
NO
NO
*p<.01, Crawford’s t-test versus CA controls (Crawford & Howell, 1998)
**p<.001, Crawford’s t-test versus CA controls (Crawford & Howell, 1998)
Current Study Part Two

41 dyslexic children (identified through SEN)
(DYS group)

42 children with same mean IWR as dyslexic
group (IW group)
Results – IW match, poor at NWR
73 controls matched to dyslexics
on irregular word reading
R= 0.85, F(72) = 199
(p<.001)
Individual 90%
confidence intervals
Falling below the lower
line reveals IWR
relative to NWR.
7/41 poor at NWR
relative to IWR.
Results – are these pure phonological cases?
Participant
IWR deficit
NWR deficit
Dissociation
Slow reading
IWR
Slow reading
NWR
PURE?
2
YES**
YES**
STRONG
YES*
YES*
NO
3
YES**
YES**
STRONG
YES**
NO
NO
6
YES**
YES**
STRONG
YES**
YES**
NO
27
YES*
YES**
STRONG
YES**
YES**
NO
34
YES**
YES**
STRONG
YES*
YES*
NO
28
YES**
YES**
STRONG
YES**
YES**
NO
41
YES**
YES**
STRONG
NO
NO
NO
*p<.01, Crawford’s t-test versus CA controls (Crawford & Howell, 1998)
**p<.001, Crawford’s t-test versus CA controls (Crawford & Howell, 1998)
Summary of results
Controls
Poor at NWR relative to
IWR
Poor at IWR relative to
NWR
CA
26
19
RL
11
3
IW / NW
7
5
Conclusions



Predictions borne out:
CA match reveals many dissociative cases
RL match reveals reduced number of dissociative cases

IW and NW matches reveal intermediary numbers of cases
not replicated in control population

Conservative estimates of prevalence (due to low overall
nonword reading and irregular word reading ability of
dyslexic group)
Conclusions

Type of control group used to assess normal performance for
a group of dyslexic children radically alters the conclusion of
which children are poor at either nonword reading or irregular
word reading

Dyslexic children can be found whose irregular word reading
is far worse than would be predicted from their nonword
reading.
The End
Thank you for listening
Thanks to all the schools and children who took part.
References
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Campbell, R. & Butterworth, B. (1985). Phonological dyslexia and dysgraphia in a highly
literate subject: A developmental case with associated deficits of phonemic processing
and awareness. The Quarterly Journal of Experimental Psychology: 37A, 435-475.
Coltheart, M., Masterson, J., Byng, S., Prior, M. & Riddoch, J. (1983). Surface dyslexia.
Quarterly Journal of Experimental Psychology: 35A, 469-495.
Crawford, J. R. & Howell, D. C. (1998). Comparing an individual’s test score against
norms derived from small samples. The Clinical Neuropsychologist, 12 (4), 482-486.
Douklias, S. D., Masterson, J. & Hanley, J. R. (2009). Surface and phonological
developmental dyslexia in Greek: Cognitive Neuropsychology, 26 (8), 705-723.
Friedmann, N., Kerbel, N. & Shvimer, L. (2010). Developmental attentional dyslexia.
Cortex: 46, 1216-1237.
Friedmann, N. & Lukov, L. (2008). Developmental surface dyslexias. Cortex, 44, 11461160. DOI: 10.1016/j.cortex.2007.09.005.
Hanley, J. R. & Gard, F. (1995). A dissociation between developmental surface and
phonological dyslexia in two undergraduate students. Neuropsychologia: 33 (7),909-914.
Hanley, J. R., Hastie, K. & Kay, J. (1992). Developmental surface dyslexia and
dysgraphia: an orthographic processing impairment. The Quarterly Journal of
Experimental Psychology: 44A, 285-319.
Marshall, J. C. (1984). Toward a rational taxonomy of the developmental dyslexias. In R.
N. Malatesha & H. A. Whitaker (Eds.), Dyslexia: A global issue. The Hague: Martinus
Nijhoff.
Masterson, J., Laxon, V. & Morag, S. (1992). Beginning reading with phonology. British
Journal of Psychology: 83, 1-12.
Rack, J. P., Snowling, M. J. & Olson, R. K. (1992). The nonword reading deficit in
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Images: Stuart Miles & Phaitoon
Wimmer, H., Mayringer, H., & Landerl, K. 2000 The double-deficit hypothesis and
difficulties in learning to read a regular orthography Journal of Educational Psychology 92
(4) 668-680
Current Study Descriptives
group
N
Age (years,
months)
WRAT
absolute
WRAT
standard
IWR
NWR
RWR
DYS group
41
12, 0
490
79
43.9
51.7
66.9
CA group
43
12, 0
509
103
88.0
91.2
95.5
RL group
61
8, 11
490
99
44.1
60.5
68.1
IW group
73
8, 11
490
99
44.0
56.8
67.4
NW group
48
8, 8
489
100
46.1
51.5
65.6