Between 1980 and 1996, the DLD was tested and evaluated in several cross-sectional and longitudinal studies. A full description of these is given in the manual.
Reliability and Internal Consistency
Inter-rater reliability was studies by measuring the Pearson correlation coefficient for the different subscales. The correlation coefficients for the different subscales varied between .44 and .94. Only for subscale ‘Behavioural disturbance’ was the correlation between raters relatively low (.44). This low correlation resulted from differences within one of the six pairs of raters. The results of the other subscales were satisfactory.
Relationship between intellectual level and DLD scores
As expected, a negative correlation was found between the intellectual level and the scores on the subscales ‘Short-term memory’, ‘Long-term memory’, ‘Orientation’, ‘Speech’, and ‘Practical skills’.
Relationship between an expert diagnosis of dementia and DLD scores
No other satisfactory assessment tools used for the assessment of dementia, evaluated for people with learning disabilities were available, therefore a specialist diagnosis by a physician and/or psychologist with expert knowledge of dementia and learning disabilities was used to judge the sensitivity of DLD scores. When scores of all individual participants were classified according to the results of a discriminant analysis, a correct diagnosis was made for an average of 72% of participants. A correct diagnosis based on DLD scores was particularly difficult in cases of severe or profound learning disability, extreme apathy or clouded consciousness.