01 NRDC Dyslexia 1-88 update - Texthelp

Developmental dyslexia in adults: a research review 165
Screening tests, whether behavioural checklists, cognitive test batteries or hybrid methods,
have one purpose: to reduce the time and expense of diagnosis. In theory, the outcome of a
screening test entails referral of true and false positives and non-referral of true and false
negatives. Tests are therefore required to be sensitive to the condition under investigation by
including as high a proportion of true positives as possible and to be specific to that condition
by excluding as high a proportion of true negatives as possible. In a rule-of-thumb fashion,
the tests serve a purpose, but it is improbable that they do so with equal efficiency. That is to
say, their predictive values are likely to differ, in that one test may yield a higher proportion
than another test of ‘test positives’ who are found to be ‘true positives’.
One complication is that the predictive value of any screening test is unlikely to be a constant.
It may vary strikingly according to the context in which it is used, so that an acceptably high
predictive value for a clinic population (where the condition screened for is relatively common
and where the test might have demonstrated its usefulness) may become an unacceptably
low predictive value when the same test is used in a general population survey (where the
condition screened for is relatively uncommon).
A further complication is that the test might have been normed in a way that makes it
unsuitable for use in an epidemiological study. For example, it might omit to take age or
maturational effects into account where there should be norms for different age-groups. Or it
might omit to take sex differences into account where there should be norms for each sex. Or
it might need variant forms to take cultural or linguistic differences into account. Where timed
tests are used, it might be that norms derived from a high-achieving sample of university
students are unsuitable for use with a sample of low-achieving young adults. This will be the
case where reaction times are assessed, where failure to take the difference into account
would refer an excessive proportion of people whose attainments, however low, could be
expected on the basis of their general ability.
What criteria would a screening instrument need to satisfy in order to offer an adequate way
of assessing the prevalence of dyslexia in an epidemiological study? A counsel of perfection
would require a predictive value so high that the instrument would be, in effect, a diagnostic
test. Unless and until such a test becomes available, a realistic set of criteria must prioritise
optimal predictive value and availability of demographically-adjusted norms. There might be a
trade-off between these two criteria.
If or when a screening instrument has been identified on realistic criteria, a decision can be
made as to whether the data obtained through its use could be entered into any analyses. It is
unlikely that any screened individuals could be identified as dyslexics for the purpose of data
analysis, for the reasons already stated. However, it is possible that self-reported behaviours
or scores on test items could be included in data analyses with no less confidence than
applies to self-report in general.
If the desired unit of analysis is the dyslexic individual, then it is essential to supplement any
screening test data. This might be done in two ways.
If the screening instrument is a behavioural checklist, the conventional wisdom would follow
it with a full-scale diagnostic assessment by an educational psychologist. The feasibility of
this procedure in a large-scale epidemiological survey would first need to be established. It is
a moot point, in any case, whether full-scale diagnostic assessment could succeed in
identifying dyslexic individuals to an acceptable level of accuracy, given the inutility of