This thesis sought to address the application of non-parametric item response theory (NIRT) to cognitive and functional assessment in dementia. Performance on psychometric tests is key to diagnosis and monitoring of dementia. NIRT can be used to improve the psychometric properties of tests used in dementia assessment in multiple ways: confirming an underlying unidimensional structure, establishing formal item hierarchical patterns of decline, increasing insight by examining item parameters such as difficulty and discrimination, and creating shorter tests. From a NIRT approach item difficulty refers to the ease with which an item is endorsed. Discrimination is an index of how well an item can differentiate between patients of varying levels of severity. Firstly I carried out a systematic review to identify applications of both parametric and non-parametric IRT to measures assessing global cognitive functioning in people with dementia. This review demonstrated that IRT can increase the interpretive power of cognitive assessment scales and confirmed the limited number of IRT analyses of cognitive scales in dementia populations. This thesis extended this approach by applying Mokken scaling analysis to commonly used measures of current cognitive ability (Addenbrooke’s Cognitive Examination-Revised (ACE-R)) and of premorbid cognitive ability (National Adult Reading Test (NART)). Differential item functioning (DIF) by diagnosis identified slight variations in the patterns of hierarchical decline in the ACE-R. These disease-specific sequences of decline could serve as an adjunct to diagnosis, for example where learning a name and address is a more difficult task than being orientated in time, late onset Alzheimer’s disease is a more probable diagnosis than mixed Alzheimer’s and vascular dementia. These analyses also allowed key items to be identified which can be used to create briefer scales (mini-ACE and Mini-NART) which have good psychometric properties. These scales are clinically relevant, comprising highly discriminatory, invariantly ordered items. They also allow sensitive measurement and adaptive testing and can reduce test administration time and patient stress. Impairment of functional abilities represents a crucial component of dementia diagnosis with performance on these functional tasks predictive of overall disease. A second aspect of this thesis, therefore, was the application of Mokken scaling analyses to measures of functional decline in dementia, specifically the Lawton Instrumental Activities of Daily Living (IADL) scale and Physical Self-Maintenance Scale (PSMS). While gender DIF was observed for several items, implying the likelihood of equal responses from men and women is not equal a generally consistent pattern of impairment in functional ability was observed across different types of dementia.

Purpose: Vocal fatigue (VF) is a primary vocal symptom experienced by professional voice users, such as teachers, whose voice is an occupational tool. The study determines the utilization of the Vocal Fatigue Index (VFI), a 19-item scale in identifying symptoms of VF and its severity in teachers. Methods: Using responses of 695 teachers who completed the VFI, Mokken scaling was conducted on the items to identify the experiences of VF and its associated hierarchical nature of VF symptoms in teachers. Mokken scaling was completed on a total of four groups: (a) Total teachers group, (b) No VF group, (c) Low VF group, and (d) High VF group. Results: Results revealed differences in item hierarchies between total teachers and across the separate groups of VF severity. Item hierarchy for teachers highlighted items from physical discomfort at the mild end of the hierarchy to items from symptom improvement with rest at the severe end of the hierarchy. Items related to avoidance presented as a separate scale for teachers presenting with high VF. Discussion: Mokken scaling in teachers provides insight into the underlying complexity of the experience of VF symptoms and reliance on differential behavioral strategies in its management, suggesting the heterogenous nature of latent trait for VF in this specific population.

Mokken (1971) 提出兩個無母數反應試題理論模型，包含單調同質性模型(MHM)和雙重單調同質性模型(DMM)，Grayson (1988) 和Huynh (1994)說明並證明出在單調同質性模型架構之下，受試者二元反應試題的回答總分與受試者的潛在特質具有MLR(monotone likelihood ratio)的性質，因此也具有SOM(stochastic ordering of the manifest variable)及SOL(stochastic ordering of the latent trait)這兩個隨機排序(stochastic ordering)的特性。另外，Mokken (1971) 也提到在Mokken量表下，受試者的試題回答總分與受試者的潛在特質有高度相關。然而這些好的特性都僅只於理論上的說法，實務的應用上並沒有實際的數字可作為使用者的參考依據。本研究將利用模擬實驗的方式，就上述議題作探討。 模擬結果顯示，未加權的時候，使用答題總分來排序受試者的潛在特質或藉由受試者的潛在特質來預估其答題總分之正確率都會隨著鑑別參數的增加而增加，前者正確率約有七成以上，後者正確率則大約有八成以上；受試者的潛在特質與其答題總分的相關係數也隨著鑑別參數增加而增加，大約介於0.50與0.80之間。 / Mokken (1971) proposed two Nonparametric Item Response Theory Models, the Monotone Homogeneity Model (MHM) and the Double Monotonicity Model (DMM). Under MHM, Grayson (1988) and Huynh (1994) showed that the unweighted total score for dichotomous items has monotone likelihood ratio (MLR) in the latent trait θ, which in turn implies two stochastic ordering (SO) properties, namely SOM (stochastic ordering of the manifest variable) and SOL (stochastic ordering of the latent trait). In addition, Mokken (1971) also mentioned that the total score were highly correlated with the latent trait for subjects. However, these properties are only theoretical arguments, and there are no actual figures that can serve as a guideline for practitioners regarding how good the properties are. We hence try to answer some of the questions through simulation experiments in this study. Simulation results show that the accuracy rate of using unweighted total score to rank the latent trait of subjects and the accuracy rate of using the latent trait to predict the total score for subjects will increase with the discrimination parameters. The former is about more than 70%, while the latter is about more than 80%. The correlation coefficients between the total score and the latent trait of subjects will also increase with the discrimination parameters, ranging between 0.50 and 0.80.

The Voice Foundation Vocal fatigue is a frequent symptom and a debilitating condition affecting individuals with voice disorders. In spite of the various attempts to define and quantify vocal fatigue, this complex trait has not been well understood. Mokken scaling was performed on the Vocal Fatigue Index (VFI) to develop a hierarchical understanding of the latent trait of vocal fatigue. Two hundred nine patients with voice disorders completed the VFI and provided the item responses necessary to complete the Mokken scaling. Results revealed a moderately strong Mokken scale and that the VFI presents a hierarchical structure to the underlying trait of vocal fatigue. Mokken scaling contributes to the ongoing investigation to the underlying construct of vocal fatigue and may provide additional information about specific complaints within the population of those with voice disorders.

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Purpose Vocal fatigue (VF) is a primary vocal symptom experienced by professional voice users, such as teachers, whose voice is an occupational tool. The study determines the utilization of the Vocal Fatigue Index (VFI), a 19-item scale in identifying symptoms of VF and its severity in teachers. Methods Using responses of 695 teachers who completed the VFI, Mokken scaling was conducted on the items to identify the experiences of VF and its associated hierarchical nature of VF symptoms in teachers. Mokken scaling was completed on a total of four groups: (a) Total teachers group, (b) No VF group, (c) Low VF group, and (d) High VF group. Results Results revealed differences in item hierarchies between total teachers and across the separate groups of VF severity. Item hierarchy for teachers highlighted items from physical discomfort at the mild end of the hierarchy to items from symptom improvement with rest at the severe end of the hierarchy. Items related to avoidance presented as a separate scale for teachers presenting with high VF. Discussion Mokken scaling in teachers provides insight into the underlying complexity of the experience of VF symptoms and reliance on differential behavioral strategies in its management, suggesting the heterogenous nature of latent trait for VF in this specific population.

Vocal fatigue is a frequent symptom and a debilitating condition affecting individuals with voice disorders. In spite of the various attempts to define and quantify vocal fatigue, this complex trait has not been well understood. Mokken scaling was performed on the Vocal Fatigue Index (VFI) to develop a hierarchical understanding of the latent trait of vocal fatigue. Two hundred nine patients with voice disorders completed the VFI and provided the item responses necessary to complete the Mokken scaling. Results revealed a moderately strong Mokken scale and that the VFI presents a hierarchical structure to the underlying trait of vocal fatigue. Mokken scaling contributes to the ongoing investigation to the underlying construct of vocal fatigue and may provide additional information about specific complaints within the population of those with voice disorders.