Objectives: Epidemiologic studies often suffer from incomplete data, measurement error (or misclassification), and confounding. Each of these can cause bias and imprecision in estimates of exposure... Show moreObjectives: Epidemiologic studies often suffer from incomplete data, measurement error (or misclassification), and confounding. Each of these can cause bias and imprecision in estimates of exposure-outcome relations. We describe and compare statistical approaches that aim to control all three sources of bias simultaneously.Study Design and Setting: We illustrate four statistical approaches that address all three sources of bias, namely, multiple imputation for missing data and measurement error, multiple imputation combined with regression calibration, full information maximum likelihood within a structural equation modeling framework, and a Bayesian model. In a simulation study, we assess the performance of the four approaches compared with more commonly used approaches that do not account for measurement error, missing values, or confounding.Results: The results demonstrate that the four approaches consistently outperform the alternative approaches on all performance metrics (bias, mean squared error, and confidence interval coverage). Even in simulated data of 100 subjects, these approaches perform well.Conclusion: There can be a large benefit of addressing measurement error, missing values, and confounding to improve the estimation of exposure-outcome relations, even when the available sample size is relatively small. (C) 2020 The Authors. Published by Elsevier Inc. Show less
Smit, R.A.J.; Trompet, S.; Dekkers, O.M.; Jukema, J.W.; Cessie, S. le 2019
This study presents an agent-based simulation model exploring the patterns of presence and absence of Late Pleistocene Neanderthals in western Europe. HomininSpace implements a parameterized... Show moreThis study presents an agent-based simulation model exploring the patterns of presence and absence of Late Pleistocene Neanderthals in western Europe. HomininSpace implements a parameterized generic demographic and social model of hominin dispersal while avoiding parameter value biases and explicitly modelled handicaps. Models are simulated through time within a high-resolution environment where reconstructed temperatures and precipitation levels influence the carrying capacity of the landscape. Model parameter values are assigned and varied automatically while optimizing the match with Neanderthal archaeology using a Genetic Algorithm (GA) inspired by the processes of natural selection. The system is able to traverse the huge parameter space that is created by the complete set of all possible parameter value combinations to find those values that will result in a simulation that matches well with archaeological data in the form of radiometrically obtained presence data. Show less
The energy system is at the heart of two of the greatest challenges of the 21st century: decreasing CO2 emissions to meet the ambitions of the Paris agreement while fulfilling the growing energy... Show moreThe energy system is at the heart of two of the greatest challenges of the 21st century: decreasing CO2 emissions to meet the ambitions of the Paris agreement while fulfilling the growing energy demand associated with the economic aspirations of a growing world population. The required energy transition has to emerge from the interaction between social actors, the economy, technology and the eco-system of our planet. That is the subject of this dissertation which explores the question how individual drivers and the interactions between actors influence the path and pace of the developing energy transition.To analyse the dynamics of interactions between actors, a relatively new modeling method, agent-based modeling, has been applied, with which actors, agents, as computer entities can be simulated. By simulating realistic actor behavior, the dynamics between individual motives and emergent collective action could be analysed.This has led to new insights; simulating investors in the electricity market for example has shown that a fully liberalized electricity market, even with strong CO2, will not ensure complete decarbonisation of the electricity system.All this has led to recommendations for public and private actors and more general conclusions about the use of models in general. Show less
This thesis examines how computer software can be used to analyse medical images of an aseptically loosening hip prosthesis, and subsequently to plan and guide a minimally invasive cement injection... Show moreThis thesis examines how computer software can be used to analyse medical images of an aseptically loosening hip prosthesis, and subsequently to plan and guide a minimally invasive cement injection procedure to stabilize the prosthesis. We addressed the detection and measurement of periprosthetic bone lesions from CT image volumes. Post-operative CTs of patients treated at our institution were analysed. We developed tissue classification algorithms that automatically label periprosthetic bone, cement and fibrous interface tissue. An existing particle-based multi-material meshing algorithm was adapted for improved Finite Element model creation. We then presented HipRFX, a proof-of-concept software tool for planning and guidance during percutaneous cement refixation procedures. Show less
Fetoscopic surgery is a surgical technique that is used to treat fetus(es) that are still inside the pregnant uterus. Coming years, more fetoscopic surgery will be performed. The most commonly... Show moreFetoscopic surgery is a surgical technique that is used to treat fetus(es) that are still inside the pregnant uterus. Coming years, more fetoscopic surgery will be performed. The most commonly performed procedure is laser surgery for twin-twin transfusion syndrome. This thesis shows learning curves for this procedure and current practice in relation to technical aspects and pregnancy outcomes. We show how to monitor performance and address specific subgroups in which laser surgery can be more complicated. Since teaching and training in fetoscopic surgery is challenging, we create and validate an evidence-based evaluation tool for the laser procedure. To conclude, we develop a standardized training curriculum with a high fidelity simulator model. Show less
Glorie, K.M.; Klerk, M. de; Wagelmans, A.P.M.; Klundert, J.J. van de; Zuidema, W.C.; Claas, F.H.J.; Weimar, W. 2013
The neutral hydrogen distribution and its evolution is closely related to various aspects of star formation. This makes understanding and modeling the HI distribution critically important for... Show moreThe neutral hydrogen distribution and its evolution is closely related to various aspects of star formation. This makes understanding and modeling the HI distribution critically important for studying galaxy evolution. The main focus of this thesis is therefore the study of the cosmic distribution of neutral hydrogen using hydrodynamical cosmological simulations. To do this, we combine hydrodynamical cosmological simulations based on the OverWhelmingly Large Simulations (OWLS; Schaye et al. 2010) with accurate radiative transfer, and account for different photoionizing processes. Then, we study the cosmic distribution of HI and its evolution. Furthermore, we study the physical connection between neutral hydrogen and galaxies by analyzing the distribution of HI absorbers with respect to galaxies close to them. Show less
Lonkhuijzen, L. van; Roosmalen, J. van; Zeeman, G. 2011
Shouder replacement surgery is difficult surgery, with a relatively large risk on limited post-operative range of motion for patients. Adaptations to the anatomy of joints by placing a prosthesis... Show moreShouder replacement surgery is difficult surgery, with a relatively large risk on limited post-operative range of motion for patients. Adaptations to the anatomy of joints by placing a prosthesis affects the articulation of the joint. In this thesis we present a software system that simulates and visualises these effects. By loading a CT-scan of the shoulder of a patient we can simulate the range of motion of the joint and visualize limitations as a result of rigid structures of the joint. Surgeons may set up an operation plan and see what the consequences of the operation will be for the range of motion of the patient. The thesis investigates aspects that are relevant for the system. We describe an algorithm to convert the scan data to bone models. In addition, a validation experiment is presented. A method for motion registration and visualisation of recorded kinematic data is presented. Finally, this thesis concerns the application of the system to different surgical problems, such as hip arthroplasty and shoulder fractures. Show less
We study several aspects of the formation of galaxies, using numerical simulations. We investigate the influence of about thirty different sub-grid physics recipes for cooling, star formation,... Show moreWe study several aspects of the formation of galaxies, using numerical simulations. We investigate the influence of about thirty different sub-grid physics recipes for cooling, star formation, supernova feedback, AGN feedback etc. on the resulting galaxy populations with large SPH simulations. We investigate several parameters that quantify the environment of galaxies and present the strongest measure of halo mass and a new parameter that is insensitive to halo mass. We look at the effects of input physics and dust attenuation on the simulated luminosity functions and compare luminosity functions directly obtained from simulations, with those using observers' tools on mock images. Regardless of most parameters used for the mock image creation and the detection of sources, these two LFs agree well. The last chapter compares several ways of sampling stellar IMFs in clusters that follow a cluster mass function in order to see how the choice of CMF and sampling method influence the resulting integrated galactic initial mass function. The effects are only significant if the CMF extends as a steep power-law down to a few solar masses. We study the effects of these IGIMFs on the galaxies' integrated photometry and metal and O-star content. Show less
Factorization methods, such as the quadratic sieve and the number field sieve, spend a lot of time on the sieving step, in which the necessary relations are collected for factoring the given number... Show moreFactorization methods, such as the quadratic sieve and the number field sieve, spend a lot of time on the sieving step, in which the necessary relations are collected for factoring the given number N. Relations are smooth or k-semismooth numbers (numbers with either all prime factors below some bound or all with the exception of at most k prime factors that do not exceed a second bound) or pairs of these type of numbers. In this thesis, we predict the amount of k-semismooth numbers needed to factor N, based on asymptotic approximation formulas (these formulas generalize the published results), and compare them with the amount of k-semismooth numbers found during the factorization of N. Furthermore, for the number field sieve we propose a method for predicting the number of necessary relations for factoring N with given parameters, and the corresponding sieving time. The basic idea is to do a small but representative amount of sieving and analyze the relations in this sample. We randomly generate relations according to the relevant distribution as observed in the sample and process these relations. Experiments show that our predictions of the number of necessary relations are within 2% of the number of relations needed in the real factorization. Show less
The biological clock regulates daily and seasonal rhythms in mammals. This clock is located in the suprachiasmatic nuclei (SCN), which are two small nuclei each consisting of 10,000 neurons. The... Show moreThe biological clock regulates daily and seasonal rhythms in mammals. This clock is located in the suprachiasmatic nuclei (SCN), which are two small nuclei each consisting of 10,000 neurons. The neurons of the SCN endogenously generate a rhythm of approximately 24 hours. Under the influence of the light-dark cycle, the SCN produce a coordinated output that is subjected to daily environmental changes. The adaptation to the light-dark cycle is a property of the neuronal network of the SCN. This neuronal network also explains the adjustment to long summer days and short winter days, and to shifts in the light-dark cycle caused by transatlantic flights or shift work. In this thesis the neuronal network of the SCN is investigated using computational techniques. The computer simulations were directed by experimental results, while, vice versa, new experiments were guided by results from the simulations. These coordinated efforts of computational science and life sciences show how properties emerge at the neuronal network level, that are not present in individual cells. Show less
