• Journal of the Korean Data and Information Science Society
• /
• v.28 no.1
• /
• pp.1-10
• /
• 2017

Longitudinal data of Korean child academic achievement have been used to find the significant exploratory variables under the assumption of independent repeated measured data. Using the exploratory variables in previous research works, we analyze the linear mixed model incorporating the fixed and random effects for child academic achievement to detect the significant exploratory variables. Korea welfare panel study data observed three times between 2006 and 2012 by additional survey for children. The child academic achievement is evaluated by the sum of academic achievements of Korean, English and Mathematics. We also investigate the multicollinearity and the missing mechanism and select some popular correlation matrices to analyze the linear mixed model.

• The Korean Journal of Applied Statistics
• /
• v.28 no.2
• /
• pp.211-219
• /
• 2015

Generalized linear mixed models are used to analyze longitudinal categorical data. Random effects specify the serial dependence of repeated outcomes in these models; however, the estimation of a random effects covariance matrix is challenging because of many parameters in the matrix and the estimated covariance matrix should satisfy positive definiteness. Several approaches to model the random effects covariance matrix are proposed to overcome these restrictions: modified Cholesky decomposition, moving average Cholesky decomposition, and partial autocorrelation approaches. We review several approaches and present potential future work.

• The Korean Journal of Applied Statistics
• /
• v.27 no.6
• /
• pp.923-932
• /
• 2014

The Hurdle model can to analyze zero-inflated count data. This model is a mixed model of the logit model for a binary component and a truncated Poisson model of a truncated count component. We propose a new hurdle model with a general heterogeneous random effects covariance matrix to analyze longitudinal zero-inflated count data using modified Cholesky decomposition. This decomposition factors the random effects covariance matrix into generalized autoregressive parameters and innovation variance. The parameters are modeled using (generalized) linear models and estimated with a Bayesian method. We use these methods to carefully analyze a real dataset.

• The Korean Journal of Applied Statistics
• /
• v.28 no.2
• /
• pp.241-250
• /
• 2015

An important scientific objective of longitudinal studies involves tracking the probability of a subject having certain health condition over the course of the study. Proper definitions and estimates of disease risk tracking have important implications in the design and analysis of long-term biomedical studies and in developing guidelines for disease prevention and intervention. We study in this paper a class of rank-tracking probabilities to describe a subject's conditional probabilities of having certain health outcomes at two different time points. Linear mixed effects models are considered to estimate the tracking probabilities and their ratios of interest. We apply our methods to an epidemiological study of childhood cardiovascular risk factors.

• Journal of the Korean Data and Information Science Society
• /
• v.26 no.6
• /
• pp.1367-1375
• /
• 2015

Consider the linear growth models for longitudinal data analysis. Several kind of linear growth models are selected such as time-effect and random-effect models as well as a dummy variable included model. In this work, simulation data are generated with normality assumption, and both binormal ROC curve and AUC are obtained and compared for various linear growth models. It is found that ROC curves have different shapes and AUC increase slowly, as values of the covariance increase and the time passes for random-effect models. On the other hand, AUC increases very fast as values of covariance decrease. When the covariance has positive value, we explored that the variances of random-effect models increase and the increment of AUC is smaller than that of AUC for time-effect models. And the increment of AUC for time-effect models is larger than the increment for random-effect models.

• KDI Journal of Economic Policy
• /
• v.15 no.4
• /
• pp.3-35
• /
• 1993

본고(本稿)의 "연간거시모형(年間巨視模型)"은 경제구조(經濟構造)가 비교적 동질적(同質的)이라 할 수 있는 1970년대 이후의 연간자료(年間資料)를 이용하여 주요총량지표(主要總量指標)의 변동을 통합(統合) 단순화(單純化)된 구조식으로 파악함으로써, 거시경제정책(巨視經濟政策) 효과분석(效果分析)과 중(中) 단기(短期) 예측에 적합하도록 개발되었다. 동(同) 모형(模型)은 주요가격변수(主要價格變數)들의 내생화와 GNP갭변수의 도입을 통해 기존모형들의 가격의 내생성(內生性) 경시 및 공급측면(供給側面) 경시적 구조를 보완하고 있으며, 80년대 이후 최근에 이르기까지 우리 경제의 거시경제변동(巨視經濟變動)을 상당히 안정적(安定的)으로 추적하는 것으로 나타났다. "연간거시모형(年間巨視模型)"에 의한 통화(通貨) 금리정책(金利政策) 효과의 분석 결과, 여(與) 수신금리(受信金利)의 인상(引上)은 실질소득(實質所得) 및 물가(物價)의 하락(下落)과 국제수지(國際收支)의 개선(改善)을 가져오는 것으로 나타났으며, 계속충격(繼續衝擊)의 경우 여(與) 수신금리(受信金利) 1%포인트의 인상은 평균 0.277%의 실질소득(實質所得) 감소효과(減少效果)를 갖는 것으로 나타났다. 계속적인 통화량(通貨量) 1%의 증가는 물가(物價)를 누적적(累積的)으로 상승시켜서 4년후에는 상승효과(上昇效果)가 0.249%에 달하게 되는 것으로 나타났다. 동(同) 모형(模型)에서는 여신금리의 인상이 경제안정화(經濟安定化) 효과(效果)를 통해 오히려 실세금리(實勢金利)의 안정(安定)에 기여하는 것으로 나타났다. 재정정책(財政政策)의 정책실험 결과, 정부소비(政府消費)의 증가로 나타나는 재정규모(財政規模) 증대의 경우 재정적자(財政赤字)를 수반하는 경우와 수반하지 않는 경우의 차이가 단적으로 나타났다. 재정규모(財政規模)의 증대(增大)는 두가지 경우 모두에 있어서 물가상승(物價上昇)을 유발하지만 그 실제적(實際的) 크기는 재정적자(財政赤字)를 수반하는 경우 훨씬 크게 나타났다. 재정규모(財政規模)의 증가(增加)가 일시적일 경우, 두가지 경우 모두에 있어 물가에 대한 정(正)의 효과(效果)는 지속적인 반면 실질소득(實質所得)에 대한 효과(效果)는 일시적인 것으로 나타났다.

• Journal of the Korean Applied Science and Technology
• /
• v.35 no.2
• /
• pp.568-577
• /
• 2018

This study was conducted to evaluate the quality characteristics. Trehalose was used to manufacture rice bread in two ways: one was to substitute trehalose for sugar by (0, 25, 50, and 75%) trehalose in each sample and the other was to analyze the temporal changes in rice bread on Day 0, on Day 2, and on Day 4. The results concerning the characteristics of specific volumes, water content, water activity, variation in weight and height, physical property test, and sensory test and measurements of the temporal changes are as follows: First, as for variation in specific volumes, rice bread containing trehalose had the content and temporal changes decrease significantly (p<0.001). Second, as for texture, a higher trehalose content led to a slightly lower level of hardness. As for the temporal changes, addition of trehalose is generally effective in decreasing hardness of rice bread. the sample containing trehalose showed an almost similar value. The control group not containing trehalose showed the significantly lowest level of cohesiveness, springiness, Chewiness. Finally, the sensory evaluations revealed that the taste, Flavor, and Sweetness was at a significantly higher level in the group with trehalose substituted for sugar than in the control group. An increase in the trehalose content led to an increase in overall eating quality.

• The Korean Journal of Applied Statistics
• /
• v.33 no.3
• /
• pp.281-296
• /
• 2020

Repeated outcomes from the same subjects are referred to as longitudinal data. Analysis of the data requires different methods unlike cross-sectional data analysis. It is important to model the covariance matrix because the correlation between the repeated outcomes must be considered when estimating the effects of covariates on the mean response. However, the modeling of the covariance matrix is tricky because there are many parameters to be estimated, and the estimated covariance matrix should be positive definite. In this paper, we consider analysis of multivariate longitudinal data via two modeling methodologies for the covariance matrix for multivariate longitudinal data. Both methods describe serial correlations of multivariate longitudinal outcomes using a modified Cholesky decomposition. However, the two methods consider different decompositions to explain the correlation between simultaneous responses. The first method uses enhanced linear covariance models so that the covariance matrix satisfies a positive definiteness condition; in addition, and principal component analysis and maximization-minimization algorithm (MM algorithm) were used to estimate model parameters. The second method considers variance-correlation decomposition and hypersphere decomposition to model covariance matrix. Simulations are used to compare the performance of the two methodologies.

• Journal of the Korean Ceramic Society
• /
• v.26 no.4
• /
• pp.588-590
• /
• 1989

Charge sensitivity and its frequency response characteristics were measured in poled and aged lead zirconate titanate(PZT) ceramics prepared by sintering. Aged PZT has lower charge sensitivity and lower mounted resonance frequency than just poled PZT.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2008.06a
• /
• pp.41-42
• /
• 2008

p-type(100) Si 위에 BTMSM과 산소를 혼합한 전구체를 가지고 PECVD 방법을 사용 하여 저유전상수를 갖는 SiOCH막을 형성하였다. 알루미늄 전극을 구현한 MIS (Al/SiOCH/p-si(100)) 구조의 커패시터를 가지고 C-V 특성을 측정하여 유전상수를 계산하였다. 상온에서 증착된 SiOCH 박막의 유전상수는 $450^{\circ}C$에서 30분 동안 열처리 후 뚜렷하게 감소하는 경향을 나타냈으며, 상온 및 대기압에서 공기 중에 노출시켜 자연 산화과정을 겪은 후에 각각의 유전상수는 전체적으로 증가하였지만, 열처리한 박막이 상대적으로 경시효과(aging effects)에 대하여 안정화된 것을 확인하였다.