| 1. |
- Dominicus, Annica, 1976-
(författare)
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Latent variable models for longitudinal twin data
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Longitudinal twin data provide important information for exploring sources of variation in human traits. In statistical models for twin data, unobserved genetic and environmental factors influencing the trait are represented by latent variables. In this way, trait variation can be decomposed into genetic and environmental components. With repeated measurements on twins, latent variables can be used to describe individual trajectories, and the genetic and environmental variance components are assessed as functions of age. This thesis contributes to statistical methodology for analysing longitudinal twin data by (i) exploring the use of random change point models for modelling variance as a function of age, (ii) assessing how nonresponse in twin studies may affect estimates of genetic and environmental influences, and (iii) providing a method for hypothesis testing of genetic and environmental variance components. The random change point model, in contrast to linear and quadratic random effects models, is shown to be very flexible in capturing variability as a function of age. Approximate maximum likelihood inference through first-order linearization of the random change point model is contrasted with Bayesian inference based on Markov chain Monte Carlo simulation. In a set of simulations based on a twin model for informative nonresponse, it is demonstrated how the effect of nonresponse on estimates of genetic and environmental variance components depends on the underlying nonresponse mechanism. This thesis also reveals that the standard procedure for testing variance components is inadequate, since the null hypothesis places the variance components on the boundary of the parameter space. The asymptotic distribution of the likelihood ratio statistic for testing variance components in classical twin models is derived, resulting in a mixture of chi-square distributions. Statistical methodology is illustrated with applications to empirical data on cognitive function from a longitudinal twin study of aging.
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| 2. |
- Stenfors, Cecilia U. D., 1982-
(författare)
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Subjective Cognitive Complaints in the Working Population : The Influence of Objective Cognitive Functioning and Working Conditions
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cognitive functioning is important for managing work and life in general. However, subjective cognitive complaints (SCC), involving self-perceived difficulties with concentration, memory, decision making, and clear thinking are common in the general and in the working population and can be coupled with both lowered well-being and work ability. The present thesis investigated the extent to which SCC among people in the work force can be explained by objective cognitive functioning (study I & II) and working conditions (study III), utilizing samples from the working population. The potential roles of other common psychological problems which often co-occur with SCC were also investigated in studies I-III.In Study I, high levels of SCC were associated with significantly poorer episodic memory performance during high executive demands and a trend was found towards poorer episodic memory, while not being associated with semantic memory. In Study II, high levels of SCC were associated with significantly poorer executive cognitive performance on all three executive cognitive tests used. Symptoms of depression, chronic stress and sleeping problems were found to play an important role in the relations between SCC and episodic memory during divided attention in study I and executive cognitive functioning in study II. In Study III, in all cross-sectional data analyses, high quantitative demands, information and communication technology (ICT) demands, underqualification in the work situation and inter-personal conflicts were positively associated with SCC, whereas social support, good resources at work and overqualification in the work situation were negatively associated with SCC. In all prospective data analyses, quantitative job demands, ICT demands and underqualification were positively associated with future SCC, including when adjusted for baseline cognitive complaints.The findings may guide prevention of and interventions for SCC among people in the work force.
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