| 1. |
- Andersson, Björn, 1984-
(författare)
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Contributions to Kernel Equating
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The statistical practice of equating is needed when scores on different versions of the same standardized test are to be compared. This thesis constitutes four contributions to the observed-score equating framework kernel equating.Paper I introduces the open source R package kequate which enables the equating of observed scores using the kernel method of test equating in all common equating designs. The package is designed for ease of use and integrates well with other packages. The equating methods non-equivalent groups with covariates and item response theory observed-score kernel equating are currently not available in any other software package.In paper II an alternative bandwidth selection method for the kernel method of test equating is proposed. The new method is designed for usage with non-smooth data such as when using the observed data directly, without pre-smoothing. In previously used bandwidth selection methods, the variability from the bandwidth selection was disregarded when calculating the asymptotic standard errors. Here, the bandwidth selection is accounted for and updated asymptotic standard error derivations are provided.Item response theory observed-score kernel equating for the non-equivalent groups with anchor test design is introduced in paper III. Multivariate observed-score kernel equating functions are defined and their asymptotic covariance matrices are derived. An empirical example in the form of a standardized achievement test is used and the item response theory methods are compared to previously used log-linear methods.In paper IV, Wald tests for equating differences in item response theory observed-score kernel equating are conducted using the results from paper III. Simulations are performed to evaluate the empirical significance level and power under different settings, showing that the Wald test is more powerful than the Hommel multiple hypothesis testing method. Data from a psychometric licensure test and a standardized achievement test are used to exemplify the hypothesis testing procedure. The results show that using the Wald test can provide different conclusions to using the Hommel procedure.
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| 2. |
- González, Rodrigo A.
(författare)
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Continuous-time System Identification : Refined Instrumental Variables and Sampling Assumptions
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Continuous-time system identification deals with the problem of building continuous-time models of dynamical systems from sampled input and output data. There are two main approaches in this field: indirect and direct. In the indirect approach, a suitable discrete-time model is first determined, and then it is transformed into continuous-time. On the other hand, the direct approach obtains a continuous-time model from the sampled data without an intermediate discrete-time model. In both approaches there exists a dichotomy between discrete-time data and continuous-time models, which can induce robustness issues and complications in the theoretical analysis of identification methods. These difficulties are addressed in this thesis.First, we consider the indirect approach to continuous-time system identification. For a zero-order hold sampling mechanism, this approach usually leads to an excess of model zeros when the true system has a relative degree greater than one. Inspired by the indirect prediction error method, we propose an indirect-approach estimator that guarantees stability in the model and enforces the desired number of poles and zeros in the continuous-time transfer function estimate.The second part of this thesis concerns the asymptotic properties and extensions of direct continuous-time identification methods. We provide a comprehensive statistical analysis of the simplified refined instrumental variable method for continuous-time systems (SRIVC), which is a widely-used direct identification algorithm that applies an adaptive prefiltering to the sampled input and output data. We prove that the SRIVC estimator is generically consistent and asymptotically efficient under some mild conditions when taking into account the intersample behavior of the signals in the analysis, and we give conditions under which these statistical properties are not achieved. An extended analysis is provided for when the model is over-parameterized. Later, we propose and analyze the statistical properties of an extension of the SRIVC estimator that can deal with input signals that cannot be interpolated exactly via hold reconstructions. The standard SRIVC estimator and its extension for arbitrary inputs, together with other refined instrumental variable methods, are also investigated in closed-loop settings and are further enhanced to deal with the identification of unstable systems.The last part of this thesis focuses on the analysis and identification of continuous-time systems subject to band-limited input excitations. The non-causal behavior of the band-limited discrete-time equivalent system is studied in detail, and the findings are later used for designing novel non-parametric and parametric identification methods for when the input is band-limited. Special treatment is given to identification with continuous-time multisine inputs. For that case, we investigate fundamental relations between prediction error methods, optimal refined instrumental variables, and interpolation and approximation of frequency response function estimates.All of the methods and theoretical results are accompanied by extensive simulation tests that verify our findings.
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| 3. |
- Jimenez-Gonzalez, Alejandro
(författare)
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Stolen genes, a shortcut to success : Evolution of metabolic and detoxification capacities in Diplomonads
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Parasites do not represent a single evolutionary lineage meaning that they have evolved several times. The changes that parasites have undergone to adapt to such a lifestyle are not entirely understood. This thesis focuses on the study of diplomonads (Fornicata, Metamonada), a group of host-associated or free-living protists, to better understand how they adapted to different environments and hosts.Diplomonads, and close relatives, are found in low-oxygen environments. However, some species can withstand fluctuating levels of oxygen. In a first study, we reconstructed the oxygen detoxification pathway of Fornicata and study its evolution. Comparative genomics showed that Fornicata shares a common pathway with lineage-specific modifications. Phylogenetic analyses showed a pathway in a constant change where proteins have been gained and lost.In a second study, the Giardia muris genome was sequenced and compared to the genome of G. intestinalis WB. We reconstructed the metabolic capacities of both species. Our analyses showed that the observed differences are the result of gene acquisitions or differential losses that can be explained based on differences in the environment of the hosts.Considering what we observed in the two previous studies, we reconstructed the metabolic capacities of four diplomonads. Using cluster analysis, we reconstructed the putative metabolism of the last Diplomonadida common ancestor. Our analyses suggested that this ancestor was, most likely, an obligate host-associated organism. However, we identified that traits associated with parasitic diplomonads evolved in a free-living ancestor.In the last study, we analyzed the genome of Hexamita inflata, a free-living, diplomonad. Our analyses showed that Trepomonas sp. PC1 and H. inflata acquired important genes for the adaptation to a secondary free-living in a common ancestor. However, our analyses also showed independent adaptations. The synthesis of glutathione and the acquisition of glutathione peroxidase, most likely, allow H. inflata to detoxify higher levels of oxygen and arsenic than other diplomonads.In conclusion, this thesis highlights the value of metabolic analyses to identify how microbial eukaryotes interact with their environment. The phylogenetic approach shows that the acquisition of genes and differential losses have been important processes in the adaptation of different hosts and environments.
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