A Nonlinear Random Coefficients Model for Degradation Testing

As an alternative to traditional life testing, degradation tests can be effective in assessing product reliability when measurements of degradation leading to failure can be observed. This article presents a degradation model for highly reliable light displays, such as plasma display panels and vacuum fluorescent displays (VFDs). Standard degradation models fail to capture the burn-in characteristics of VFDs, when emitted light actually increases up to a certain point in time before it decreases (or degrades) continuously. Random coefficients are used to model this phenomenon in a nonlinear way, which allows for a nonmonotonic degradation path. In many situations, the relative efficiency of the lifetime estimate is improved over the standard estimators based on transformed linear models

Weighted Rank Regression with Dummy Variables for Analyzing Accelerated Life Testing Data

In this article, we propose a new rank regression model to extrapolate the product lifetimes at normal operation environment from accelerated testing data. Weighted least squares method is used to compensate for nonconstant error variance in the regression model. A group of dummy variables is incorporated to check model adequacy. We also developed a customizing software for quick-and-easy implementation of the method so that reliability engineers can easily exploit it. Simulation studies show that, under light censoring, the proposed method performs comparatively well in predicting the lifetimes even with small sample sizes. With its computational ease and graphical presentation, the proposed method is expected to be more popular among reliability engineers

Degradation Models

Reliability testing typically generates product lifetime data, but for some tests, covariate information about the wear and tear on the product during the life test can provide additional insight into the product’s lifetime distribution. This usage, or degradation, can be the physical parameters of the product (e.g., corrosion thickness on a metal plate) or merely indicated through product performance (e.g., the luminosity of a light emitting diode). The measurements made across the product’s lifetime are degradation data, and degradation analysis is the statistical tool for providing inference about the lifetime distribution from the degradation data

On Data Depth and the Application of Nonparametric Multivariate Statistical Process Control Charts

The purpose of this article is to summarize recent research results for constructing nonparametric multivariate control charts with main focus on data depth based control charts. Data depth provides data reduction to large-variable problems in a completely nonparametric way. Several depth measures including Tukey depth are shown to be particularly effective for purposes of statistical process control in case that the data deviates normality assumption. For detecting slow or moderate shifts in the process target mean, the multivariate version of the EWMA is generally robust to non-normal data, so that nonparametric alternatives may be less often required

A Superposed Log-Linear Failure Intensity Model for Repairable Artillery Systems

This article investigates complex repairable artillery systems that include several failure modes. We derive a superposed process based on a mixture of nonhomogeneous Poisson processes in a minimal repair model. This allows for a bathtub-shaped failure intensity that models artillery data better than currently used methods. The method of maximum likelihood is used to estimate model parameters and construct confidence intervals for the cumulative intensity of the superposed process. Finally, we propose an optimal maintenance policy for repairable systems with bathtub-shaped intensity and apply it to the artillery-failure data

Degradation Models and Implied Lifetime Distributions

In experiments where failure times are sparse, degradation analysis is useful for the analysis of failure time distributions in reliability studies. This research investigates the link between a practitioner\u27s selected degradation model and the resulting lifetime model. Simple additive and multiplicative models with single random effects are featured. Results show that seemingly innocuous assumptions of the degradation path create surprising restrictions on the lifetime distribution. These constraints are described in terms of failure rate and distribution classes

Statistical Models for Hot Electron Degradation in Nano-Scaled MOSFET Devices

In a MOS structure, the generation of hot carrier interface states is a critical feature of the item\u27s reliability. On the nano-scale, there are problems with degradation in transconductance, shift in threshold voltage, and decrease in drain current capability. Quantum mechanics has been used to relate this decrease to degradation, and device failure. Although the lifetime, and degradation of a device are typically used to characterize its reliability, in this paper we model the distribution of hot-electron activation energies, which has appeal because it exhibits a two-point discrete mixture of logistic distributions. The logistic mixture presents computational problems that are addressed in simulation

MMP-Inhibitory Effects of Flavonoid Glycosides from Edible Medicinal Halophyte Limonium tetragonum

Limonium tetragonum has been well-known for its antioxidative properties as a halophyte. This study investigated the antimetastasis effect of solvent-partitioned L. tetragonum extracts (LTEs) and isolated compounds on HT1080 mouse melanoma cell model with a focus on matrix metalloproteinase (MMP) activity and TIMP and MAPK pathways. Upregulation and stimulation of MMPs result in elevated degradation of extracellular matrix which is part of several complications such as metastasis, cirrhosis, and arthritis. The anti-MMP capacity of LTEs was confirmed by their MMP-inhibitory effects, regulation of MMP and TIMP expression, and suppression of MAPK pathway. Among all tested LTEs, 85% aq. MeOH and n-BuOH were found to be most active fractions which later yielded two known flavonoid glycosides, myricetin 3-galactoside and quercetin 3-o-beta-galactopyranoside. Anti-MMP potential of the compounds was confirmed by their ability to regulate MMP expression through inhibited MAPK pathway activation. These results suggested that L. tetragonum might serve as a potential source of bioactive substances with effective anti-MMP properties

Dysnatremia, its correction, and mortality in patients undergoing continuous renal replacement therapy: a prospective observational study

This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.Abstract Background Although dysnatremia has been reported to be correlated with mortality risk, this issue remains unresolved in patients undergoing continuous renal replacement therapy (CRRT). Furthermore, it has not been determined whether change in or correction of sodium is related to mortality risk in this subset. Methods A total of 569 patients were prospectively enrolled at the start of CRRT between May 2010 and September 2013. The patients were divided into 5 groups: normonatremia (135–145 mmol/L), mild hyponatremia (131.1–134.9 mmol/L), moderate to severe hyponatremia (115.4–131.0 mmol/L), mild hypernatremia (145.1–148.4 mmol/L), and moderate to severe hypernatremia (148.5–166.0 mmol/L). The non-linear relationship between sodium and mortality was initially explored. Subsequently, the odds ratios (ORs) for 30-day mortality were calculated after adjustment of multiple covariates. Results The relationship between baseline sodium and mortality was U-shaped. The mild hyponatremia, moderate to severe hyponatremia, and moderate to severe hypernatremia groups had greater ORs for mortality (1.65, 1.91, and 2.32, respectively) than the normonatremia group (all P values < 0.05). However, later sodium levels (24 and 72 h after CRRT) did not predict 30-day mortality. Furthermore, the changes in sodium over 24 or 72 h, including the appropriate correction of dysnatremia, did not show any relationships with mortality, irrespective of baseline sodium level. Conclusions Sodium level at the start of CRRT was a strong predictor of mortality. However, changes in sodium level and the degree of sodium correction were not associated with the mortality risk in the patients with CRRT

Influence of Radiation Dose to Reconstructed Breast Following Mastectomy on Complication in Breast Cancer Patients Undergoing Two-Stage Prosthetic Breast Reconstruction

Purpose: This study investigated the association between radiation dose and complication rate in patients who underwent breast reconstruction to understand the role of radiation hypofractionated regimen, boost radiation therapy (RT), and RT techniques.Methods: We retrospectively evaluated 75 patients treated with post-mastectomy adjuvant RT for breast cancer in the setting of two-stage prosthetic breast reconstruction. Near maximum radiation dose (Dmax) in the 2 or 0.03 cc of reconstructed breast or overlying breast skin was obtained from dose-volume histograms.Results: Post-RT complications occurred in 22.7% of patients. Receiver operating characteristic analysis showed that all near Dmax parameters were able to predict complication risk, which retained statistical significance after adjusting other variables (odds ratio 1.12 per Gy, 95% confidence interval 1.02–1.23) with positive dose-response relationship. In multiple linear regression model (R2 = 0.92), conventional fractionation (β = 11.7) and 16 fractions in 2.66 Gy regimen (β = 3.9) were the major determinants of near Dmax compared with 15 fractions in 2.66 Gy regimen, followed by utilization of boost RT (β = 3.2). The effect of bolus and dose inhomogeneity seemed minor (P &gt; 0.05). The location of hot spot was not close to the high density metal area of the expander, but close to the surrounding areas of partially deflated expander bag.Conclusions: This study is the first to demonstrate a dose-response relationship between risk of complications and near Dmax, where hypofractionated regimen or boost RT can play an important role. Rigorous RT-quality assurance program and modification of dose constraints could be considered as a critically important component for ongoing trials of hypofractionation. Based on our findings, we initiated a multi-center retrospective study (KROG 18-04) and a prospective study (NCT03523078) to validate our findings