Intervention for Dyslexia - The British Dyslexia Association

According to Wolfendale and Bryans (1979), the chief criteria for screening instruments
for use in education are:
1. Tests should not be lengthy or elaborate.
2. The collected data should be readily and routinely available within the school.
3. The data should be related to the goals and processes of the school.
4. The teaching methods to be used following identification should be clearly
thought out.
It should come as no surprise that there is a trade-off between the two requirements for
ease of administration and accuracy of results. Relatively coarse procedures are not, in
general, very accurate, and rarely are accurate procedures simple or easy to administer.
The principal task confronting anyone attempting to create a screening device for use in
education, therefore, is to find a satisfactory compromise between practicality and
accuracy: what Singleton (1997a) has referred to as the ‘practicality-accuracy dilemma’.
When educationalists have talked about screening it has not always been clear what
type of screening is being referred to. Screening may be broadly divided into two types:
classificatory screening and predictive screening. The latter has sometimes been
referred to as ‘speculative screening’ (Potton, 1983). In classificatory screening an
existing condition or difficulty is identified, while in predictive screening, a condition or
difficulty that has yet to become apparent is predicted from its antecedents.
Psychologists and educationists have long recognised the danger of screening results
shaping or reinforcing teachers’ expectation of pupils (see Jansky, 1977; Kingslake,
1982; Streiner, 2003). In particular, any screening device will produce a proportion of
children who are incorrectly classified. Such misclassification of children may lead to
inappropriate action and unrealistic presumptions on the part of the teacher. The
danger, of course, is that the true predictive accuracy of a screening device may not
properly be known because the results of screening invite action on the part of teachers,
and any intervention is likely to have some effect on the phenomena being predicted.
Consequently, before any screening device should be accepted for general use, its
accuracy should be properly established by means of a prospective validation study,
which should be carried out in the absence of intervention. In education, however,
rarely have these principles been applied, and all too often screening tests have been
championed solely on the twin virtues of faith and face validity.
4.2.3 The accuracy of screening instruments
In theory, screening tests should meet the fundamental requirements of all
psychological assessment instruments, in the sense that they should be objective and
standardised measures of behaviour, the reliability and validity of which must be
assured. They should also be norm-referenced or criterion-referenced in a manner that
satisfactorily meets all psychometric criteria. Over and above such preliminaries,
however, the paramount question must be: how accurate are they at predicting/
identifying the target group, in this case, children with dyslexia? In statistical terms,
screening instruments are binary classification tests: that is, they assign individuals to
one or other of two categories, e.g. ‘dyslexic’ or ‘non-dyslexic’ (exceptionally they might
also have a third category ‘possibly, or borderline, dyslexic’).
Correlation and discriminant function analysis are the two statistical techniques most
frequently used to evaluate screening instruments. The correlation coefficient indicates
Intervention for Dyslexia 81