Department of Psychology, Queens College and the Graduate Centre, City University of New York, New York, USA
Department of Dental Public Health and Oral Health Services Research, GKT Dental Institute, Kings College London
Estia Centre, Guys Hospital, London, UK
Correspondence: Professor Nick Bouras, MHiLD, York Clinic, 47 Weston Street, Guys Hospital, London SE1 3RR, UK. E-mail: nick.bouras{at}kcl.ac.uk
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ABSTRACT |
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Aims To report data on the psychometric properties of the Psychiatric Assessment Schedule for Adults with Developmental Disabilities (PASADD) Checklist.
Method The PASADD Checklist was completed for 226 adults as part of the assessment process for a specialist mental health service for people with intellectual disabilities.
Results Internal consistency was acceptable. Factor analysis revealed one main factor that was characterised by items related to mood. The Checklist was sensitive to differences between diagnostic groups and had an overall sensitivity of 66%; its specificity was 70%.
Conclusions The PASADD Checklist is a quick and easy to use screening tool. Although at present it is the best measure available, it should not be the only method used to identify psychiatric disorders in people with intellectual disabilities.
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INTRODUCTION |
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METHOD |
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Procedure
Data collection
Each individual attended an assessment interview with a psychiatrist at
which information including clinical history and current psychiatric diagnosis
was recorded. In addition, a key informant such as a relative or staff member
was asked to complete the PASADD Checklist for each individual. The
assessing psychiatrist was masked to the PASADD Checklist score at
assessment.
Data analysis
Data analysis was conducted using the Statistical Package for the Social
Sciences, version 10. Four analyses were conducted. First, in order to look at
reliability, item analyses of each of the five scales (AE) and the
three total scores (13) were conducted and Cronbachs was
calculated. Alpha values greater than 0.7 are considered acceptable
(Nunnally, 1978). Itemtotal point biserial correlations were also calculated to measure
internal consistency. Rogue items, which correlated with a total score less
than 0.3, were identified. Second, an exploratory factor analysis of the
PASADD Checklist items was conducted, in order to assess if any items
in the Checklist were measuring aspects of the same underlying dimensions or
factors. A principal components analysis with quartimax rotation was used. The
number of factors was determined using a scree plot of the variances before
rotation. Third, to assess validity, PASADD Checklist scores were
compared with clinical psychiatric diagnoses. To make a valid comparison
between each diagnosis, only those diagnoses present in more than ten cases
were included. Diagnoses fulfilling this criterion were no psychiatric
diagnosis, schizophrenia spectrum disorder, personality disorder,
anxiety disorder, depressive disorder and adjustment reaction. Oneway analysis
of variance (ANOVA) with post hoc Scheffé tests were used.
Finally, in order to examine the sensitivity and specificity of the Checklist,
a summary of the numbers of people who crossed any PASADD Checklist
threshold, in relation to the numbers who had a clinical psychiatric diagnosis
covered by the Checklist, was calculated.
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RESULTS |
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Factor analysis
The results of the factor analysis are shown in
Table 3. The first nine factors
had eigen-values greater than 1.0 and these factors accounted for 64% of the
variance. An examination of a scree plot suggested a single factor structure,
since the first factor accounted for 20% of the variance and the subsequent
eight factors accounted for 48% of the variance. The first factor was
characterised primarily by items related to mood, such as loss of interest and
energy, sadness, avoiding conversation, low self-esteem, loss of appetite and
confidence, and poor concentration. The second factor was characterised by
three items related to sleep disturbance. The third factor was characterised
by three items related to psychotic symptoms. It was difficult to characterise
subsequent factors clearly.
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Validity
Table 4 shows the
PASADD Checklist scores for people who were diagnosed by a clinician as
having no diagnosis, schizophrenia spectrum disorder,
personality disorder, anxiety disorder, depressive disorder or adjustment
reaction. There was a significant difference between individuals on total
score 1 (affective/neurotic disorder), in which people who had depressive
disorder scored higher than those with no diagnosis, and all other psychiatric
diagnoses. There was a significant difference between individuals on total
score 2 (possible organic disorder), in which those with depressive disorder
scored higher than those with no diagnosis. There was a significant difference
between individuals on total score 3 (psychotic disorder), in which people
with schizophrenia spectrum disorder scored higher than those with no
psychiatric diagnosis and all other diagnoses.
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Sensitivity and specificity
Table 5 shows the numbers of
people who crossed any PASADD Checklist threshold in relation to the
numbers who had a clinical psychiatric diagnosis covered by the Checklist. The
sensitivity of the PASADD Checklist was 66% and the specificity was
70%.
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DISCUSSION |
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The number of rogue items is perhaps to be expected, as the checklist was not designed to identify specific disorders but rather to indicate the possible presence of a range of psychiatric disorders. There is thus some variation in the items included in each scale or total score to reflect the range of disorders.
Factor structure
Nine factors were initially identified, accounting for 64% of the variance.
The first three factors, characterised by mood items, sleep disturbance and
psychotic symptoms, are similar to three of the factors identified by the
authors of the Checklist, which they characterise as depression, restlessness
and psychosis (Moss et al,
1998). The other factors, however, are hard to characterise and
account for little of the variance in this study.
Validity
The validity of the PASADD Checklist appears to be good when
considering the scores of people who have different psychiatric diagnoses.
People who had depressive disorder scored higher on total score 1
(affective/neurotic disorder) than those who did not have this disorder,
demonstrating that in terms of affective/neurotic disorders the Checklist
performed well and identified the correct people. Individuals with depressive
disorder also scored significantly higher than those without this disorder on
total score 2 (possible organic disorder), although the significance was
relatively low. This is not surprising, because there is some overlap between
the scales that contribute to total score 1 and total score 2. Also, no
individual in this section of the analysis had an organic disorder, so we
would not expect the scores of the people with the disorders that are included
to vary significantly on this organic disorder threshold.
People with schizophrenia spectrum disorder scored significantly higher on total score 3 (psychotic disorder) than people with any other diagnosis, confirming that the Checklist performs well on this disorder.
Sensitivity
Any screening tool must be assessed in relation to sensitivity. The main
criticism of the PASADD Checklist in this study relates to this
measure.
The sensitivity (proportion of people with a psychiatric disorder covered by the Checklist who are correctly classified by the instrument as having a psychiatric disorder) of the PASADD Checklist was 66%. This is lower than the figure of 78% calculated from the findings of the developers of the Checklist (Moss et al, 1998) and is also lower than other screening measures such as the 12-item General Health Questionnaire, which has a sensitivity of 76% (Goldberg et al, 1997). There are several possible explanations for the presence of false negatives. Moss et al (1998) found that the likelihood of crossing the thresholds rose with severity of the illness. Although in our study the severity of clinician diagnosis was not recorded, it might have been the case that some of these people did not have symptoms that were severe enough to be picked up by the Checklist. Of the people not crossing any threshold, 14 had schizophrenia spectrum disorder, which is a chronic disorder. At assessment these peoples symptoms might have been absent and therefore not identified by the Checklist, if controlled through medication or if the persons disorder was in remission. Unfortunately, these data were not available, so this can only be proposed as a possible explanation.
The large number of people diagnosed as having an affective disorder but not crossing any of the thresholds (n=25) might be due to the nature of these diagnoses. Although some aspects may be observable, and the Checklist focuses mainly on these elements, scoring highly on the checklist and crossing a threshold is reliant to some extent on the person being able to communicate how he or she is feeling. This may be easier to elicit from people with intellectual disabilities in a clinical assessment rather than by use of a Checklist that is not completed by the patients themselves.
The breakdown of level of intellectual disability in those who had a diagnosis covered by the Checklist but who did not cross the threshold was similar to the breakdown of the total sample.
Although the above explanations may very well be valid, the data are not available to prove them, and the fact remains that the sensitivity of the PASADD Checklist in this study was fairly low. A further consideration raised by this analysis is that 14% of this sample had a psychiatric diagnosis that the PASADD Checklist was not designed to identify and therefore could not be expected to pick up.
If anything, we would expect a screening instrument to be overinclusive rather than underinclusive. In this study 15% of the total sample had no psychiatric disorder, or a psychiatric disorder that was not covered by the Checklist but crossed at least one of its thresholds. This is higher than the 8% of false positives calculated from findings of the Checklists developers (Moss et al, 1998). For the purpose of screening people for further psychiatric assessment, it is preferable to have false positives rather than false negatives: people with intellectual disability may find going to a psychiatric out-patient clinic very upsetting and a high rate of false positives would be costly. Therefore we would hope for a low false positive rate. The specificity of the PASADD Checklist was 70%, indicating that 70% of people who did not have a psychiatric disorder or had a psychiatric disorder that was not covered by the PASADD Checklist were correctly identified.
We did not explore the sensitivity and false positive rates of the PASADD Checklist with lower threshold scores. However, this may be something to consider in the future.
In summary, the PASADD Checklist had acceptable internal consistency, one main factor characterised by mood items was sensitive to differences between diagnostic groups, and had an overall sensitivity of 66%.
Limitations of the study
There was only a small number of people with an organic disorder in this
sample. Consequently, it was difficult to determine how successful the
PASADD Checklist was at identifying these disorders. It would also have
been useful to have had some measure of the severity of the disorders as
clinically diagnosed, as this would have enabled us to comment further on the
issue of severity of symptoms affecting the crossing of the threshold
scores.
The PASADD Checklist has been revised since our study was completed, and although the items in the two versions differ only slightly, there is some variation in the order the items are presented. Whether this revision will affect the PASADD Checklists psychometric properties remains to be seen in future research.
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Clinical Implications and Limitations |
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LIMITATIONS
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REFERENCES |
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Moss, S., Patel, P., Prosser, H., et al (1993) Psychiatric morbidity in older people with moderate and severe learning disability. I: Development and reliability of the patient interview (PASADD). British Journal of Psychiatry, 163, 471 480.[Abstract]
Moss, S., Prosser, H., Costello, H., et al (1998) Reliability and validity of the PASADD Checklist for detecting psychiatric disorders in adults with intellectual disability. Journal of Intellectual Disability Research, 42, 173 183.[CrossRef][Medline]
Nunnally, J. C. (1978) Psychometric Theory (2nd edn). New York: McGraw-Hill.
Sturmey, P., Reed, J. & Corbett, J. (1993) Assessment of psychiatric disorders in people with learning difficulties: a psychometric review of available measures. Psychological Medicine, 21, 143 155.
Received for publication July 14, 2003. Revision received October 25, 2004. Accepted for publication October 30, 2004.
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