• Journal of the Korean Data and Information Science Society
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• 제27권3호
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• pp.815-825
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• 2016

Marginalized random effects models (MREMs) are often used to analyze longitudinal categorical data. The models permit direct estimation of marginal mean parameters and specify the serial correlation of longitudinal categorical data via the random effects. However, it is not easy to estimate the random effects covariance matrix in the MREMs because the matrix is high-dimensional and must be positive-definite. To solve these restrictions, we introduce two modeling approaches of the random effects covariance matrix: partial autocorrelation and the modified Cholesky decomposition. These proposed methods are illustrated with the real data from Korean genomic epidemiology study.

• Journal of the Korean Data and Information Science Society
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• 제28권4호
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• pp.927-936
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• 2017

To analyze longitudinal count data, Poisson linear mixed models are commonly used. In the models the random effects covariance matrix explains both within-subject variation and serial correlation of repeated count outcomes. When the random effects covariance matrix is assumed to be misspecified, the estimates of covariates effects can be biased. Therefore, we propose reasonable and flexible structures of the covariance matrix using autoregressive and moving average Cholesky decomposition (ARMACD). The ARMACD factors the covariance matrix into generalized autoregressive parameters (GARPs), generalized moving average parameters (GMAPs) and innovation variances (IVs). Positive IVs guarantee the positive-definiteness of the covariance matrix. In this paper, we use the ARMACD to model the random effects covariance matrix in Poisson loglinear mixed models. We analyze epileptic seizure data using our proposed model.

• 터널과지하공간
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• 제18권5호
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• pp.338-342
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• 2008

기존의 연구에서 터널 종단변형도는 대부분 수치해석적 방법으로 구해졌으나, 본 논문에서는 실제 계측치를 이용하여 종단변형도를 구성하고 굴착에 따른 터널의 종단변위 발생 경향을 검토하였다. 막장전방의 변위는 수평경사계 계측치의 최대변위에 대한 지점 변위를 비교 분석하여 가장 적합한 결과를 나타내는 지점의 변위를 채택하였고, 막장 후방은 천단변위계에 의한 계측치를 회귀분석하여 구하였다. 연구 대상 터널의 계측치로 천단변위를 분석한 결과, 막장 3D 이전부터 변위가 발생하였고, 막장에서는 수치해석적으로 구한 내공 변위에 비해 10%, 정도 높은 약 40%의 변위가 발생하였으며, 막장 후방 4D 이후 최종변위에 수렴하는 것으로 나타났다.

• 비파괴검사학회지
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• 제36권2호
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• pp.138-148
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• 2016

Measurement of elastic constants is crucial for engineering aspects of predicting the behavior of materials under load as well as structural health monitoring of material degradation. Ultrasonic velocity measurement for material properties has been broadly used as a nondestructive evaluation method for material characterization. In particular, pulse-echo method has been extensively utilized as it is not only simple but also effective when only one side of the inspected objects is accessible. However, the conventional technique in this approach measures longitudinal and shear waves individually to obtain their velocities. This produces a set of two data for each measurement. This paper proposes a simultaneous sensing system of longitudinal waves and shear waves for elastic constant measurement. The proposed system senses both these waves simultaneously as a single overlapped signal, which is then analyzed to calculate both the ultrasonic velocities for obtaining elastic constants. Therefore, this system requires just half the number of data to obtain elastic constants compared to the conventional individual measurement. The results of the proposed simultaneous measurement had smaller standard deviations than those in the individual measurement. These results validate that the proposed approach improves the efficiency and reliability of ultrasonic elastic constant measurement by reducing the complexity of the measurement system, its operating procedures, and the number of data.

• 유통과학연구
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• 제20권11호
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• pp.89-98
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• 2022

Purpose: Increasing competition in the higher education sector has prompted universities to enhance their marketing efforts and understand their potential customers. The study aims to explore how information sources are used and changed among prospective Vietnamese students during the decision-making process. Research Design, Data, and Methodology: This study undertakes a longitudinal study involving multiple rounds of data collection to better understand the decision-making process of prospective students. Data was collected from 12th -grade students in Vietnam through two rounds of quantitative surveys with 251 students and one round of qualitative interviews, spanning the duration of their senior year. The three stages of the decision-making process correspond to the three stages of pre-purchase period. Results: Most students decide that attending open days, taking career assessments, and looking up information online are the most important information sources to consider. The WOM sources are more important in the early stages, while university-generated sources and events are important in the later stages. Conclusion: Implications from this study may contribute to the design of more effective marketing communications campaigns as university marketers gain a better understanding of the distribution of information sources utilized for each specific stage of the decision-making process.

• Journal of Preventive Medicine and Public Health
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• 제48권2호
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• pp.111-117
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• 2015

Objectives: While epidemiologic research indicates that the prevalence of risk-taking behaviors including cigarette smoking among young people with asthma is substantial, the longitudinal patterns of cigarette smoking in this vulnerable population have received little attention. The aim of this study was to evaluate differences in the longitudinal trajectories of cigarette use behaviors from adolescence to adulthood between young people with and without asthma. Methods: Data from the National Longitudinal Study of Adolescent to Adult Health (Add Health) during the years 1994 to 1995 (Wave I, adolescence), 2001 to 2002 (Wave III, young adulthood), and 2007 to 2008 (Wave IV, adulthood) were analyzed (n=12 244). Latent growth curve models were used to examine the longitudinal trajectories of cigarette use behaviors during the transition to adulthood according to asthma status. Results: Regardless of asthma status, the trajectory means of cigarette use behaviors were found to increase, and then slightly decrease from adolescence to adulthood. In total participants, there were no statistically significant differences in initial levels and changes in cigarette use behaviors according to asthma status. However, in select sex and race subgroups (i.e., females and non-whites), former asthmatics showed greater escalation in cigarette use behaviors than did non-asthmatics or current asthmatics. Conclusions: This study indicated that the changing patterns of cigarette use behaviors during the transition to adulthood among young people with asthma are comparable to or even more drastic than those among young people without asthma.

• 자동차안전학회지
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• 제12권2호
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• pp.33-39
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• 2020

This paper presents an Around View Monitoring (AVM) stop-line detection based longitudinal position correction algorithm for automated driving on urban roads. Poor positioning accuracy of low-cost GPS has many problems for precise path tracking. Therefore, this study aims to improve the longitudinal positioning accuracy of low-cost GPS. The algorithm has three main processes. The first process is a stop-line detection. In this process, the stop-line is detected using Hough Transform from the AVM camera. The second process is a map matching. In the map matching process, to find the corrected vehicle position, the detected line is matched to the stop-line of the HD map using the Iterative Closest Point (ICP) method. Third, longitudinal position of low-cost GPS is updated using a corrected vehicle position with Kalman Filter. The proposed algorithm is implemented in the Robot Operating System (ROS) environment and verified on the actual urban road driving data. Compared to low-cost GPS only, Test results show the longitudinal localization performance was improved.

• 자동차안전학회지
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• 제12권1호
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• pp.15-25
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• 2020

This paper presents a longitudinal control algorithm for ensuring takeover time of autonomous vehicle using V2V communication. In the autonomous driving of more than level 3, autonomous systems should control the vehicles by itself partially. However if the driver's intervention is required for functional safety, the driver should take over the control reasonably. Autonomous driving system has to be designed so that drivers can take over the control from autonomous vehicle reasonably for driving safety. In this study, control algorithm considering takeover time has been developed based on computation method of takeover time. Takeover time is analysed by conditions of longitudinal velocity of preceding vehicle in time-velocity plane. In addition, desired clearance is derived based on takeover time. The performance evaluation of the proposed algorithm in this study was conducted using 3D vehicle model with actual driving data in Matlab/Simulink environment. The results of the performance evaluation show that the longitudinal control algorithm can control while securing takeover time reasonably.

• Journal of Positioning, Navigation, and Timing
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• 제6권4호
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• pp.159-166
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• 2017

The position of autonomous driving vehicle is basically acquired through the global positioning system (GPS). However, GPS signals cannot be received in tunnels. Due to this limitation, localization of autonomous driving vehicles can be made through sensors mounted on them. In particular, a 3D Light Detection and Ranging (LIDAR) system is used for longitudinal position error correction. Few feature points and structures that can be used for localization of vehicles are available in tunnels. Since lanes in the road are normally marked by solid line, it cannot be used to recognize a longitudinal position. In addition, only a small number of structures that are separated from the tunnel walls such as sign boards or jet fans are available. Thus, it is necessary to extract usable information from tunnels to recognize a longitudinal position. In this paper, fire hydrants and evacuation guide lights attached at both sides of tunnel walls were used to recognize a longitudinal position. These structures have highly distinctive reflectivity from the surrounding walls, which can be distinguished using LIDAR reflectivity data. Furthermore, reflectivity information of tunnel walls was fused with the road surface reflectivity map to generate a synthetic reflectivity map. When the synthetic reflectivity map was used, localization of vehicles was able through correlation matching with the local maps generated from the current LIDAR data. The experiments were conducted at an expressway including Maseong Tunnel (approximately 1.5 km long). The experiment results showed that the root mean square (RMS) position errors in lateral and longitudinal directions were 0.19 m and 0.35 m, respectively, exhibiting precise localization accuracy.

• 한국인구학
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• 제29권3호
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• pp.159-188
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• 2006

본 연구의 목적은 통계청에서 생산하고 있는 가구단위 조사 중 반복횡단조사로 설계된 경제 활동인구조사와 도시가계조사의 표본가구관리명부 자료를 이용하여 1998년에서 2002년까지 패널자료를 구축하고, 패널의 생존기간과 패널무응답 집단의 사회인구학적인 특성을 분석하는 것이다. 경활의 표본가구관리명부 자료를 월별 패널로 구축한 결과, 모두 응답한 가구는 46%였다. 반면에 분기별 패널로 구축된 도시가계에서 모두 응답한 가구는 34.%로 경활보다는 낮았다. 콕스비례위험모형을 통해 가구와 가구원의 사회경제적 특성이 패널의 생존기간에 미친 영향을 분석한 결과 개인차원에서는 연령, 가구차원에서는 주택소유형태에 따라 체계적인 표본소실이 발생하고 있었다. 개인의 특성별로는 여자보다는 남자가, 장년층보다는 젊은층의 패널소실 위험율이 높았다. 학력이 높을수록 패널소실확률도 함께 증가했으며, 취업자보다는 실업자의 패널소실 확률이 높았다.