• Journal of Korean Society for Quality Management
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• v.36 no.1
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• pp.31-39
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• 2008

This paper proposes a method of estimating the lifetime distribution at use condition for constant stress accelerated life tests when an infant-mortality failure mode as well as wear-out one exists. General limited failure population model is introduced to describe these failure modes. It is assumed that the log lifetime of each failure mode follows a location-scale distribution and a linear relation exists between the location parameter and the stress. An estimation procedure using the expectation and maximization algorithm is proposed. Specific formulas for Weibull distribution are obtained. An illustrative example and the simulation results are given.

• Journal of the military operations research society of Korea
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• v.17 no.1
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• pp.61-83
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• 1991

There has been a great deal of research dealing with the optimal replacement of stochastically deteriorating equipment and research in queueing systems with a finite calling population. However. there has been a lack of research which combines these two areas to deal with optimal replacement for a fixed number of machines and a limited number of repairmen. In this research, an optimal control policy for replacement involving two machines and one repairman is developed by investigating a class of age replacement policies in the context of controlling a G/M/1 queueing system with a finite calling population. The control policy to be imposed on this problem is an age-dependent control policy, described by the control limit $t^{\ast}$. The control limit is operational only when the repairman is idle; that is. if both machines are working, as soon as a machine reaches the age $t^{\ast}$ it is taken out of service for replacememt. We obtain the ${\epsilon}$-optimal control age which will minimize the long-run average system cost. An algorithm is developed that is applicable to general failure time distributions and cost functions. The algorithm does not require the condition of unimodality for implementation.

• Tuberculosis and Respiratory Diseases
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• v.83 no.2
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• pp.147-156
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• 2020

Background: Limited studies have been performed to assess readmission following hospitalization for community-acquired pneumonia (CAP) in an Asian population. We evaluated the rates, reasons, and risk factors for 30-day readmission following hospitalization for CAP in the general adult population of Korea. Methods: We performed a retrospective observational study of 1,021 patients with CAP hospitalized at Yeungnam University from March 2012 to February 2014. The primary end point was all-cause hospital readmission within 30 days following discharge after the initial hospitalization. Hospital readmission was classified as pneumonia-related or pneumonia-unrelated readmission. Results: During the study period, 862 patients who survived to hospital discharge were eligible for inclusion and among them 72 (8.4%) were rehospitalized within 30 days. In the multivariable analysis, pneumonia-related readmission was associated with para/hemiplegia, malignancy, pneumonia severity index class ≥4 and clinical instability ≥1 at hospital discharge. Comorbidities such as chronic lung disease and chronic kidney disease, treatment failure, and decompensation of comorbidities were associated with the pneumonia-unrelated 30-day readmission rate. Conclusion: Rehospitalizations within 30 days following discharge were frequent among patients with CAP. The risk factors for pneumonia-related and -unrelated readmission were different. Aspiration prevention, discharge at the optimal time, and close monitoring of comorbidities may reduce the frequency of readmission among patients with CAP.