This study focused on the relationship between the asymmetry of initial motor symptoms of Parkins... more This study focused on the relationship between the asymmetry of initial motor symptoms of Parkinson's disease (PD) and premorbid handedness of patients. Structural equation modeling has been used for this purpose. The survey consisting of validated items measuring handedness and questions related to side of occurrence of initial symptoms was administered to 472 patients with PD [277 men, 195 women, mean age 66.5 (9.3), mean duration of the disease 10 (6.1) years]. The unidimensional model of handedness fits the data well (v 2 5 37.86, df 5 20, P 5 0.009, Root Mean Square Error of Approximation 5 0.044, Comparative Fit Index 5 1.00, Standardized Root Mean Square Residual 5 0.042) and side of initial motor symptoms is not significantly related to the factor of handedness (r 50.11, SE 5 0.07, P 5 0.14). In contrast to several other studies, the results indicate that the side of first occurrence of PD signs cannot be predicted from premorbid handedness of patients.
The dimensionality and reliability of the Motor Section of the Unified Parkinson Disease Rating S... more The dimensionality and reliability of the Motor Section of the Unified Parkinson Disease Rating Scale (UPDRS III) was studied with non-parametric Mokken scale analysis. UPDRS measures were obtained on 147 patients with PD (96 men, 51 women, mean age 61, range 35-80 yrs). Mokken scale analysis revealed a four-dimensional structure of the UPDRS III. Left-sided bradykinesia and rigidity appeared to co-occur with axial signs, gait disturbance, and speech/hypomimia, whereas right-sided bradykinesia and rigidity formed a second scale. Two further small scales were found consisting of right- and left-sided tremor. Results from the scale analysis reveal that all four subscales are strong. The reliability of the two tremor scales is low because they only contain three and four items, respectively.
Background. An argument often used to support the view that psychotic experiences (PEs) in genera... more Background. An argument often used to support the view that psychotic experiences (PEs) in general population samples are a valid phenotype for studying the aetiology of schizophrenia is that risk factors for schizophrenia show similar patterns of association with PEs. However, PEs often co-occur with depression, and no study has explicitly tested whether risk factors for schizophrenia are shared between PEs and depression, or are psychopathology specific, while jointly modelling both outcomes.
Psychotic phenomena are common in the general population but are excluded from diagnostic criteri... more Psychotic phenomena are common in the general population but are excluded from diagnostic criteria for mild to moderate depression and anxiety despite their co-occurrence and shared risk factors. We used item response theory modelling to examine whether the co-occurrence of depressive, anxiety and psychotic phenomena is best explained by: (1) a single underlying factor; (2) two separate, uncorrelated factors; (3) two separate yet linked factors; or (4) two separate domains along with an underlying 'common mental distress' (CMD) factor. We defined where, along any latent continuum, the psychopathological items contributed most information. We performed a secondary analysis of cross-sectional, item-level information from measures of depression, anxiety and psychotic experiences in 6617 participants aged 13 years from the Avon Longitudinal Study of Parents and Children (ALSPAC) birth cohort and 977 participants aged 18 years from the ROOTS schools-based sample. We replicated results from one sample in the other and validated the latent factors against an earlier parental measure of mental state. In both cohorts depression, anxiety and psychotic items were best represented as a bi-factor model with a single, unitary CMD factor on which psychotic items conveyed information about the more severe end (model 4); residual variation remained for psychotic items. The CMD factor was significantly associated with the prior parental measure. Psychotic phenomena co-occur with depression and anxiety in teenagers and may be a marker of severity in a single, unitary dimension of CMD. Psychotic phenomena should be routinely included in epidemiological assessments of psychiatric morbidity, otherwise the most severe symptomatology remains unmeasured.
Background. An argument often used to support the view that psychotic experiences (PEs) in genera... more Background. An argument often used to support the view that psychotic experiences (PEs) in general population samples are a valid phenotype for studying the aetiology of schizophrenia is that risk factors for schizophrenia show similar patterns of association with PEs. However, PEs often co-occur with depression, and no study has explicitly tested whether risk factors for schizophrenia are shared between PEs and depression, or are psychopathology specific, while jointly modelling both outcomes.
Background: Mokken scaling techniques are a useful tool for researchers who wish to construct uni... more Background: Mokken scaling techniques are a useful tool for researchers who wish to construct unidimensional tests or use questionnaires that comprise multiple binary or polytomous items. The stochastic cumulative scaling model offered by this approach is ideally suited when the intention is to score an underlying latent trait by simple addition of the item response values. In our experience, the Mokken model appears to be less well-known than for example the (related) Rasch model, but is seeing increasing use in contemporary clinical research and public health. Mokken's method is a generalisation of Guttman scaling that can assist in the determination of the dimensionality of tests or scales, and enables consideration of reliability, without reliance on Cronbach's alpha. This paper provides a practical guide to the application and interpretation of this non-parametric item response theory method in empirical research with health and well-being questionnaires. Methods: Scalability of data from 1) a cross-sectional health survey (the Scottish Health Education Population Survey) and 2) a general population birth cohort study (the National Child Development Study) illustrate the method and modeling steps for dichotomous and polytomous items respectively. The questionnaire data analyzed comprise responses to the 12 item General Health Questionnaire, under the binary recoding recommended for screening applications, and the ordinal/polytomous responses to the Warwick-Edinburgh Mental Well-being Scale. Results and conclusions: After an initial analysis example in which we select items by phrasing (six positive versus six negatively worded items) we show that all items from the 12-item General Health Questionnaire (GHQ-12)when binary scoredwere scalable according to the double monotonicity model, in two short scales comprising six items each (Bech's "well-being" and "distress" clinical scales). An illustration of ordinal item analysis confirmed that all 14 positively worded items of the Warwick-Edinburgh Mental Well-being Scale (WEMWBS) met criteria for the monotone homogeneity model but four items violated double monotonicity with respect to a single underlying dimension. Software availability and commands used to specify unidimensionality and reliability analysis and graphical displays for diagnosing monotone homogeneity and double monotonicity are discussed, with an emphasis on current implementations in freeware.
War experiences (WE) and postwar environments (PWE) are associated with mental ill-health. The pr... more War experiences (WE) and postwar environments (PWE) are associated with mental ill-health. The present study aims to investigate the pathways from WE and PWE to mental ill-health and to define opportunities for intervention through analysis of the war-affected youths study (WAYS) cohort study. WAYS is an ongoing study of a large cohort of former child soldiers being conducted in Uganda. Mental health problems, subjective WE and PWE contexts were assessed by local adaptations of internationally developed measures for use with former child soldiers at least 6 years after the end of the war. Structural equation modeling was used to test two mediation hypotheses: (1) the 'trauma model' in which WE directly influence long-term mental health and (2) the 'psychosocial…
This study focused on the relationship between the asymmetry of initial motor symptoms of Parkins... more This study focused on the relationship between the asymmetry of initial motor symptoms of Parkinson's disease (PD) and premorbid handedness of patients. Structural equation modeling has been used for this purpose. The survey consisting of validated items measuring handedness and questions related to side of occurrence of initial symptoms was administered to 472 patients with PD [277 men, 195 women, mean age 66.5 (9.3), mean duration of the disease 10 (6.1) years]. The unidimensional model of handedness fits the data well (v 2 5 37.86, df 5 20, P 5 0.009, Root Mean Square Error of Approximation 5 0.044, Comparative Fit Index 5 1.00, Standardized Root Mean Square Residual 5 0.042) and side of initial motor symptoms is not significantly related to the factor of handedness (r 50.11, SE 5 0.07, P 5 0.14). In contrast to several other studies, the results indicate that the side of first occurrence of PD signs cannot be predicted from premorbid handedness of patients.
The dimensionality and reliability of the Motor Section of the Unified Parkinson Disease Rating S... more The dimensionality and reliability of the Motor Section of the Unified Parkinson Disease Rating Scale (UPDRS III) was studied with non-parametric Mokken scale analysis. UPDRS measures were obtained on 147 patients with PD (96 men, 51 women, mean age 61, range 35-80 yrs). Mokken scale analysis revealed a four-dimensional structure of the UPDRS III. Left-sided bradykinesia and rigidity appeared to co-occur with axial signs, gait disturbance, and speech/hypomimia, whereas right-sided bradykinesia and rigidity formed a second scale. Two further small scales were found consisting of right- and left-sided tremor. Results from the scale analysis reveal that all four subscales are strong. The reliability of the two tremor scales is low because they only contain three and four items, respectively.
Background. An argument often used to support the view that psychotic experiences (PEs) in genera... more Background. An argument often used to support the view that psychotic experiences (PEs) in general population samples are a valid phenotype for studying the aetiology of schizophrenia is that risk factors for schizophrenia show similar patterns of association with PEs. However, PEs often co-occur with depression, and no study has explicitly tested whether risk factors for schizophrenia are shared between PEs and depression, or are psychopathology specific, while jointly modelling both outcomes.
Psychotic phenomena are common in the general population but are excluded from diagnostic criteri... more Psychotic phenomena are common in the general population but are excluded from diagnostic criteria for mild to moderate depression and anxiety despite their co-occurrence and shared risk factors. We used item response theory modelling to examine whether the co-occurrence of depressive, anxiety and psychotic phenomena is best explained by: (1) a single underlying factor; (2) two separate, uncorrelated factors; (3) two separate yet linked factors; or (4) two separate domains along with an underlying 'common mental distress' (CMD) factor. We defined where, along any latent continuum, the psychopathological items contributed most information. We performed a secondary analysis of cross-sectional, item-level information from measures of depression, anxiety and psychotic experiences in 6617 participants aged 13 years from the Avon Longitudinal Study of Parents and Children (ALSPAC) birth cohort and 977 participants aged 18 years from the ROOTS schools-based sample. We replicated results from one sample in the other and validated the latent factors against an earlier parental measure of mental state. In both cohorts depression, anxiety and psychotic items were best represented as a bi-factor model with a single, unitary CMD factor on which psychotic items conveyed information about the more severe end (model 4); residual variation remained for psychotic items. The CMD factor was significantly associated with the prior parental measure. Psychotic phenomena co-occur with depression and anxiety in teenagers and may be a marker of severity in a single, unitary dimension of CMD. Psychotic phenomena should be routinely included in epidemiological assessments of psychiatric morbidity, otherwise the most severe symptomatology remains unmeasured.
Background. An argument often used to support the view that psychotic experiences (PEs) in genera... more Background. An argument often used to support the view that psychotic experiences (PEs) in general population samples are a valid phenotype for studying the aetiology of schizophrenia is that risk factors for schizophrenia show similar patterns of association with PEs. However, PEs often co-occur with depression, and no study has explicitly tested whether risk factors for schizophrenia are shared between PEs and depression, or are psychopathology specific, while jointly modelling both outcomes.
Background: Mokken scaling techniques are a useful tool for researchers who wish to construct uni... more Background: Mokken scaling techniques are a useful tool for researchers who wish to construct unidimensional tests or use questionnaires that comprise multiple binary or polytomous items. The stochastic cumulative scaling model offered by this approach is ideally suited when the intention is to score an underlying latent trait by simple addition of the item response values. In our experience, the Mokken model appears to be less well-known than for example the (related) Rasch model, but is seeing increasing use in contemporary clinical research and public health. Mokken's method is a generalisation of Guttman scaling that can assist in the determination of the dimensionality of tests or scales, and enables consideration of reliability, without reliance on Cronbach's alpha. This paper provides a practical guide to the application and interpretation of this non-parametric item response theory method in empirical research with health and well-being questionnaires. Methods: Scalability of data from 1) a cross-sectional health survey (the Scottish Health Education Population Survey) and 2) a general population birth cohort study (the National Child Development Study) illustrate the method and modeling steps for dichotomous and polytomous items respectively. The questionnaire data analyzed comprise responses to the 12 item General Health Questionnaire, under the binary recoding recommended for screening applications, and the ordinal/polytomous responses to the Warwick-Edinburgh Mental Well-being Scale. Results and conclusions: After an initial analysis example in which we select items by phrasing (six positive versus six negatively worded items) we show that all items from the 12-item General Health Questionnaire (GHQ-12)when binary scoredwere scalable according to the double monotonicity model, in two short scales comprising six items each (Bech's "well-being" and "distress" clinical scales). An illustration of ordinal item analysis confirmed that all 14 positively worded items of the Warwick-Edinburgh Mental Well-being Scale (WEMWBS) met criteria for the monotone homogeneity model but four items violated double monotonicity with respect to a single underlying dimension. Software availability and commands used to specify unidimensionality and reliability analysis and graphical displays for diagnosing monotone homogeneity and double monotonicity are discussed, with an emphasis on current implementations in freeware.
War experiences (WE) and postwar environments (PWE) are associated with mental ill-health. The pr... more War experiences (WE) and postwar environments (PWE) are associated with mental ill-health. The present study aims to investigate the pathways from WE and PWE to mental ill-health and to define opportunities for intervention through analysis of the war-affected youths study (WAYS) cohort study. WAYS is an ongoing study of a large cohort of former child soldiers being conducted in Uganda. Mental health problems, subjective WE and PWE contexts were assessed by local adaptations of internationally developed measures for use with former child soldiers at least 6 years after the end of the war. Structural equation modeling was used to test two mediation hypotheses: (1) the 'trauma model' in which WE directly influence long-term mental health and (2) the 'psychosocial…
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Papers by J. Stochl