Essays on aggregate and individual consumption fluctuations

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Economics, 2006.Includes bibliographical references.This thesis consists of three essays on aggregate and individual consumption fluctuations. Chapter 1 develops a quantitative model to explore aggregate and individual consumption dynamics when the income process exhibits regime-switching features, and compares its performance with the conventional linear model. For this purpose, I consider an economy populated by a large number of consumers whose incomes are subject to both aggregate and idiosyncratic shocks. The notable element of the model is that a latent regime-switching stochastic variable governs both the trend growth of the aggregate component and the counter-cyclical variances of the idiosyncratic components in individual earnings. I demonstrate that the model can provide a reasonable description of the cyclical behavior of actual consumption fluctuations, and can successfully replicate some key empirical properties of aggregate consumption growth, such as smaller volatility than income growth, greater volatility in recessions than in expansions, and a negatively skewed and leptokurtic distribution, while the typical linear model fails to do so.(cont.) The model highlights that the interaction between aggregate and idiosyncratic shocks, shifts in the cross-sectional distribution of individual consumers' optimal consumption decisions, and learning about the underlying business-cycle regime play a critical role in explaining aggregate consumption dynamics. In addition, comparison between individual and aggregate consumption and aggregation issues are addressed. Finally, I show that the regime-switching features, combined with heteroskedastic income risk, can account for more than 50% of aggregate consumption fluctuations, but less than 4% of individual consumption fluctuations. It is widely believed that utility maximization implies that expected consumption growth should be higher in recessions which are associated with higher income uncertainty because consumers with precautionary saving motives save more to tilt up their consumption path. Evidence in the literature, however, does not seem to support this prediction. Chapter 2 tries to reconcile these seemingly contradictory observations. First, noting that recessions are times of both higher income uncertainty and lower income growth, I perform comparative experiments to see each effect on expected consumption growth.(cont.) Higher income uncertainty indeed increases expected consumption growth, while lower income growth does the opposite. Next, in a calibrated switching regime income process example, where recessions are associated with lower income growth and higher uncertainty, I show the net effect may well decrease, rather than increase, expected consumption growth. I then compare my results to the usual argument in the literature, based on approximations to the Euler equation. Chapter 3 develops econometric methods to estimate consumers' risk aversion and time discount rate parameters in a dynamic stochastic general equilibrium (DSGE) model set-up, using the simulated method of moments (SMM) technique. This approach, based on a numerical solution algorithm, offers several advantages over traditional methods, which directly estimate a (linearized) Euler equation. In particular, the model allows us to incorporate a possibly non-linear underlying income process and the selection of moment conditions into the estimation procedure. I also consider two extensions by (1) allowing for the parameters of the model to be state-dependent and (2) incorporating the agent's learning about the latent aggregate state.by Youngjin Hwang.Ph.D

Effectiveness of Electrical Heating for Improved Thermal Insulation of a Multi-layered Winter Clothing System

This paper investigated the impact of the distance of the heating unit from the body in a multi-layered winter clothing system on effective thermal insulation and heating efficiency. To identify changes in the thermal insulation and heating efficiency of electrical heating in different layers inside a winter clothing ensemble, a series of thermal manikin tests was conducted. A multi-layered winter ensemble with and without activation of a heating unit was tested on the thermal manikin under two different ambient temperature conditions (10°C and 5°C). Results show that the effective thermal insulation of test ensembles increased by 5-7 percent with the activation of the heating unit compared to that without the activation. The closer the heating unit to the body, the higher the effective thermal insulation was in both ambient temperature conditions. This trend was more significant at lower ambient temperature

Facile Synthesis of Porous Silicon Nanofibers by Magnesium Reduction for Application in Lithium Ion Batteries

We report a facile fabrication of porous silicon nanofibers by a simple three-stage procedure. Polymer/silicon precursor composite nanofibers are first fabricated by electrospinning, a water-based spinning dope, which undergoes subsequent heat treatment and then reduction using magnesium to be converted into porous silicon nanofibers. The porous silicon nanofibers are coated with a graphene by using a plasma-enhanced chemical vapor deposition for use as an anode material of lithium ion batteries. The porous silicon nanofibers can be mass-produced by a simple and solvent-free method, which uses an environmental-friendly polymer solution. The graphene-coated silicon nanofibers show an improved cycling performance of a capacity retention than the pure silicon nanofibers due to the suppression of the volume change and the increase of electric conductivity by the graphene.Korea Institute of Energy Technology Evaluation and Planning (Human Resources Development Program Grant 20144030200600

A simple and efficient numerical method for the Allen–Cahn equation on effective symmetric triangular meshes

In this paper, we propose a novel, simple, efficient, and explicit numerical method for the Allen–Cahn (AC) equation on effective symmetric triangular meshes. First, we compute the net vector of all vectors starting from each node point to its one-ring neighbor vertices and virtually adjust the neighbor vertices so that the net vector is zero. Then, we define the values at the virtually adjusted nodes using linear and quadratic interpolations. Finally, we define a discrete Laplace operator on triangular meshes. We perform several computational experiments to demonstrate the performance of the proposed numerical method for the Laplace operator, the diffusion equation, and the AC equation on triangular meshes

Epidemics of enterovirus infection in Chungnam Korea, 2008 and 2009

Previously, we explored the epidemic pattern and molecular characterization of enteroviruses isolated in Chungnam, Korea from 2005 to 2006. The present study extended these observations to 2008 and 2009. In this study, enteroviruses showed similar seasonal prevalent pattern from summer to fall and age distribution to previous investigation. The most prevalent month was July: 42.9% in 2008 and 31.9% in 2009. The highest rate of enterovirus-positive samples occurred in children < 1-year-old-age. Enterovirus-positive samples were subjected to sequence determination of the VP1 region, which resolved the isolated enteroviruses into 10 types in 2008 (coxsackievirus A4, A16, B1, B3, echovirus 6, 7, 9, 11, 16, and 30) and 8 types in 2009 (coxsackievirus A2, A4, A5, A16, B1, B5, echovirus 11, and enterovirus 71). The most prevalent enterovirus serotype in 2008 and 2009 was echovirus 30 and coxsackievirus B1, respectively, whereas echovirus 18 and echovirus 5 were the most prevalent types in 2005 and 2006, respectively. Comparison of coxsackievirus B1 and B5 of prevalent enterovirus type in Korea in 2009 with reference strains of each same serotype were conducted to genetic analysis by a phylogenetic tree. The sequences of coxsackievirus B1 strains segregated into four distinct clusters (A, B, C, and D) with some temporal and regional sub-clustering. Most of Korean coxsackievirus B1 strains in 2008 and 2009 were in cluster D, while only "Kor08-CVB1-001CN" was cluster C. The coxsackievirus B5 strains segregated in five distinct genetic groups (clusters A-E) were supported by high bootstrap values. The Korean strains isolated in 2001 belonged to cluster D, whereas Korean strains isolated in 2005 and 2009 belonged to cluster E. Comparison of the VP1 amino acid sequences of the Korean coxsackievirus B5 isolates with reference strains revealed amino acid sequence substitutions at nine amino acid sequences (532, 562, 570, 571, 576-578, 582, 583, and 585)

Impact of successful restoration of sinus rhythm in patients with atrial fibrillation and acute heart failure: Results from the Korean Acute Heart Failure registry

Background: Restoring and maintaining sinus rhythm (SR) in patients with atrial fibrillation (AF) failed to show superior outcomes over rate control strategies in prior randomized trials. However, there is sparse data on their outcomes in patients with acute heart failure (AHF).Methods: From December 2010 to February 2014, 5,625 patients with AHF from 10 tertiary hospitals were enrolled in the Korean Acute Heart Failure registry, including 1,961 patients whose initial electrocardiogram showed AF. Clinical outcomes of patients who restored SR by pharmacological or electrical cardioversion (SR conversion group, n = 212) were compared to those of patients who showed a persistent AF rhythm (AF persistent group, n = 1,662).Results: All-cause mortality both in-hospital and during the follow-up (median 2.5 years) were significantly lower in the SR conversion group than in the AF persistent group after adjustment for risk factors (adjusted hazard ratio [HR]; 95% confidence interval [CI] = 0.26 [0.08–0.88], p = 0.031 and 0.59 [0.43–0.82], p = 0.002, for mortality in-hospital and during follow-up, respectively). After 1:3 propensity score matching (SR conversion group = 167, AF persistent group = 501), successful restoration of SR was associated with lower all-cause mortality (HR [95% CI] = 0.68 [0.49–0.93], p = 0.015), heart failure rehospitalization (HR [95% CI] = 0.66 [0.45–0.97], p = 0.032), and composite of death and heart failure rehospitalization (HR [95% CI] = 0.66 [0.51–0.86], p = 0.002).Conclusions: Patients with AHF and AF had significantly lower mortality in-hospital and during follow-up if rhythm treatment for AF was successful, underscoring the importance of restoring SR in patients with AHF

External knowledge search, innovative performance and productivity in the Korean ICT sector

In recent years, Korea's Information and Communication Technology (ICT) industry has grown rapidly. The prosperity of the ICT industry has brought growing recognition that rising productivity and innovative performance have made major contributions to ensuring competitive advantage in international markets. In this context, recent studies stress the importance of external knowledge for improving both innovative performance and productivity. This paper empirically investigates the effects of firms' external knowledge search behavior on their productivity as well as their innovative performance in the Korean ICT manufacturing sector. Based on firm-level data from the Science and Technology Policy Institute (STEPI), this study indicates that incremental innovation and productivity are both related to external knowledge search.External knowledge search Innovative performance Productivity Information and communication technology Korea

Modelling Weblog Success: Case of Korea

Weblogs have received attention as new, personalized media. Yet, only few of them attract public attention and become successful. This study explores weblog success factors in three categories: content creation, content features and content diffusion. During the process of content creation in weblogs, we argue that weblog service providers (WSPs) support bloggers’ resource collection. We also presume that the volume or the quality of posts in weblogs could be matter to gain visitors’ attention when weblog content (i.e., post) is generated. During the process of content diffusion, we assume that use of blogging technologies (BTs) such as trackback or RSS would enhance content-sharing activities between weblogs. Based on the data from a sample of Korean individual weblogs, our analysis indicates that weblog success (in terms of the number of unique visitors per week) is related to the WSP’s support level for content creation as well as content features