1,721,059 research outputs found
A distribution-free phase-II CUSUM procedure for monitoring service quality
No full textA new single distribution-free Phase-II CUSUM procedure based on the Cucconi test statistic for simultaneously monitoring shifts in the unknown location and scale parameters of a process is proposed. The procedure does not require the assumption of normal data. Control limits are tabulated for practical implementation of the procedure. The in-control and out-of-control performance of the procedure are comprehensively examined in terms of mean, median, variability and some percentiles of the corresponding run length distribution. A thorough comparison with the Shewhart-type procedures based on the Cucconi and Lepage statistics as well as with the CUSUM procedure based on the Lepage statistic is presented. The proposed procedure is illustrated by analysing the service quality of the Vancouver City Call Centre
Distribution-Free Lepage Type Circular Grid Charts for Joint Monitoring of Location and Scale Parameters of a Process
No full textIn the last 5 years, research works on distribution-free (nonparametric) process monitoring have registered a phenomenal
growth. A Google Scholar database search on early September 2015 reveals 246 articles on distribution-free control charts
during 2000–2009 and 466 articles in the following years. These figures are about 1400 and 2860 respectively if the word
‘nonparametric’ is used in place of ‘distribution-free’. Distribution-free charts do not require any prior knowledge about
the process parameters. Consequently, they are very effective in monitoring various non-normal and complex processes.
Traditional process monitoring schemes use two separate charts, one for monitoring process location and the other for
process scale. Recently, various schemes have been introduced to monitor the process location and process scale
simultaneously using a single chart. Performance advantages of such charts have been clearly established. In this
paper, we introduce a new graphical device, namely, circular-grid charts, for simultaneous monitoring of process
location and process scale based on Lepage-type statistics. We also discuss general form of Lepage statistics and show
that a new modified Lepage statistic is often better than the traditional of Lepage statistic. We offer a new and
attractive post-signal follow-up analysis. A detailed numerical study based on Monte-Carlo simulations is performed,
and some illustrations are provided. A clear guideline for practitioners is offered to facilitate the best selection of
charts among various alternatives for simultaneous monitoring of location-scale. The practical application of the charts
is illustrated
A class of percentile modified Lepage-type tests
No full textThe two-sample problem usually tests for a difference in location. However, there are many situations, for example in biomedicine, where jointly testing for difference in location and variability may be more appropriate. Moreover, heavy-tailed data, outliers and small-sample sizes are common in biomedicine and in other fields. These considerations make the use of nonparametric methods more appealing than parametric ones. The aim of the paper is to contribute to the literature about nonparametric simultaneous location and scale testing. More precisely, several existing tests are generalized and unified, and a new class of tests based on the Mahalanobis distance between the percentile modified test statistics for location and scale differences is introduced. The asymptotic distributions of the test statistics are obtained, and small-sample size behaviour of the tests is studied and compared to other tests via Monte Carlo simulations. It is shown that the proposed class of tests performs well when there are differences in both location and variability. A practical application is presented.The two-sample problem usually tests for a difference in location. However, there are many situations, for example in biomedicine, where jointly testing for difference in location and variability may be more appropriate. Moreover, heavy-tailed data, outliers and small-sample sizes are common in biomedicine and in other fields. These considerations make the use of nonparametric methods more appealing than parametric ones. The aim of the paper is to contribute to the literature about nonparametric simultaneous location and scale testing. More precisely, several existing tests are generalized and unified, and a new class of tests based on the Mahalanobis distance between the percentile modified test statistics for location and scale differences is introduced. The asymptotic distributions of the test statistics are obtained, and small-sample size behaviour of the tests is studied and compared to other tests via Monte Carlo simulations. It is shown that the proposed class of tests performs well when there are differences in both location and variability. A practical application is presented
Monitoring delivery time using a multiaspect distribution-free method
No full textIn the field of food service logistics, monitoring pizza delivery time is an important issue. The importance of timely pizza deliveries mirrors broader societal expectations of instant gratification and time efficiency. In this paper, we propose a new approach for designing distribution-free monitoring methods which is based on the nonparametric combination of dependent tests framework (NPC), borrowed from hypothesis testing and permutation testing theory. We analyze the application of NPC to combine several tests in order to address different aspects of the problem or viewpoints of the data. Studying the corresponding multi-aspect NPC process monitoring schemes, we discovered that combining only linear rank tests leads to a distribution-free NPC methods, whereas combining at least one non linear rank test may lead to a non distribution-free NPC method. One of the most interesting results has been obtained with the NPC Wilcoxon–Van der Waerden process monitoring method. This method takes full advantage of the flexibility of the NPC framework performing well under a wide variety of process distributions. The methods are illustrated by analyzing a real data set
Exact simultaneous location-scale tests for two shifted exponential samples
Full text linkThe failure time distribution for various items often follows a shifted (two-parameter) expo- nential model and not the traditional (one-parameter) exponential model. The shifted exponen- tial is very useful in practice, in particular in the engineering, biomedical sciences and industrial quality control when modeling time to event or survival data. The open problem of simultane- ous testing for differences in origin and scale parameters of two shifted exponential distributions is addressed. Two exact tests are proposed using maximum likelihood estimators. They are based on the combination of two statistics following a maximum-type and a distance-type ap- proach. The exact null distributions of the respective test statistics are derived analytically. Small sample type-one error rate and power of the tests are studied numerically. We showed that the test based on the maximum type combination (the Max test) should be preferred being generally more powerful than the test based on the distance type combination (the Distance test). An application to a biomedical experiment is discussed
Interpoint distance tests for high-dimensional comparison studies
No full textModern data collection techniques allow to analyze a very large number of endpoints. In biomedical research, for example, expressions of thousands of genes are commonly measured only on a small number of subjects. In these situations, traditional methods for comparison studies are not applicable. Moreover, the assumption of normal distribution is often questionable for high-dimensional data, and some variables may be at the same time highly correlated with others. Hypothesis tests based on interpoint distances are very appealing for studies involving the comparison of means, because they do not assume data to come from normally distributed populations and comprise tests that are distribution free, unbiased, consistent, and computationally feasible, even if the number of endpoints is much larger than the number of subjects. New tests based on interpoint distances are proposed for multivariate studies involving simultaneous comparison of means and variability, or the whole distribution shapes. The tests are shown to perform well in terms of power, when the endpoints have complex dependence relations, such as in genomic and metabolomic studies. A practical application to a genetic cardiovascular case-control study is discussed
Surface barrier discharges for Escherichia coli biofilm inactivation: Modes of action and the importance of UV radiation
No full textCold plasma generated in air at atmospheric pressure is an extremely effective antimicrobial agent, with proven efficacy against clinically relevant bacterial biofilms. The specific mode of bacterial inactivation is highly dependent upon the configuration of the plasma source used. In this study, the mode of microbial inactivation of a surface barrier discharge was investigated against Escherichia coli biofilms grown on polypropylene coupons. Different modes of exposure were considered and it was demonstrated that the long-lived reactive species created by the plasma are not solely responsible for the observed microbial inactivation. It was observed that a synergistic interaction occurs between the plasma generated long-lived reactive species and ultraviolet (UV) photons, acting to increase the antimicrobial efficacy of the approach by an order of magnitude. It is suggested that plasma generated UV is an important component for microbial inactivation when using a surface barrier discharge; however, it is not through the conventional pathway of direct DNA damage, rather through the synergistic interaction between liquid in the biofilm matrix and long-lived chemical species created by the discharge.</jats:p
Remediation of heavy metal contamination of sediments and soils using ligand-coated dense nanoparticles.
Full text linkSediment and soil contamination with toxic heavy metals, including cadmium (Cd2+) and lead (Pb2+), represents a major long-term remediation challenge. Resuspension of contaminated sediments into the water column, or the uptake of toxic metals from top soil, can lead to exposure of aquatic or terrestrial organisms, followed by bioconcentration, bioaccumulation and biomagnification, which may pose a threat to public health. We have developed a novel nanoscale engineered material, namely ligand-coated dense nanoparticles (Ligand DNPs), which contain a dense WO3 nanoparticle core and a shell functionalized with a metal-binding organic ligand (EDTA), to effectively sequester heavy metal ions deeper into the soil and sediments. We demonstrate that one application of Ligand DNPs can remove from 60% to almost 80% of the Cd and Pb in two different soil matrices, driving these metal ions deeper into the sediment or soil column via gravity, and making them less bioavailable. Ligand DNPs can provide a relatively fast, convenient, and efficient in-situ approach for the remediation of sediments and soils contaminated with heavy metals
Direct and indirect effects of different types of microplastics on freshwater prey (Corbicula fluminea) and their predator (Acipenser transmontanus).
Full text linkWe examined whether environmentally relevant concentrations of different types of microplastics, with or without PCBs, directly affect freshwater prey and indirectly affect their predators. Asian clams (Corbicula fluminea) were exposed to environmentally relevant concentrations of polyethylene terephthalate (PET), polyethylene, polyvinylchloride (PVC) or polystyrene with and without polychlorinated biphenyls (PCBs) for 28 days. Their predators, white sturgeon (Acipenser transmontanus), were exposed to clams from each treatment for 28 days. In both species, we examined bioaccumulation of PCBs and effects (i.e., immunohistochemistry, histology, behavior, condition, mortality) across several levels of biological organization. PCBs were not detected in prey or predator, and thus differences in bioaccumulation of PCBs among polymers and biomagnification in predators could not be measured. One of the main objectives of this study was to test the hypothesis that bioaccumulation of PCBs would differ among polymer types. Because we could not answer this question experimentally, a bioaccumulation model was run and predicted that concentrations of PCBs in clams exposed to polyethylene and polystyrene would be greater than PET and PVC. Observed effects, although subtle, seemed to be due to microplastics rather than PCBs alone. For example, histopathology showed tubular dilation in clams exposed to microplastics with PCBs, with only mild effects in clams exposed to PCBs alone
Variation of iron redox kinetics and its relation with molecular composition of standard humic substances at circumneutral pH.
No full textOxidation and reduction kinetics of iron (Fe) and proportion of steady-state Fe(II) concentration relative to total dissolved Fe (steady-state Fe(II) fraction) were investigated in the presence of various types of standard humic substances (HS) with particular emphasis on the photochemical and thermal reduction of Fe(III) and oxidation of Fe(II) by dissolved oxygen (O2) and hydrogen peroxide (H2O2) at circumneutral pH (pH 7-8). Rates of Fe(III) reduction were spectrophotometrically determined by a ferrozine method under the simulated sunlight and dark conditions, whereas rates of Fe(II) oxidation were examined in air-saturated solution using luminol chemiluminescence technique. The reduction and oxidation rate constants were determined to substantially vary depending on the type of HS. For example, the first-order rate constants varied by up to 10-fold for photochemical reduction and 7-fold for thermal reduction. The degree of variation in Fe(II) oxidation was larger for the H2O2-mediated reaction compared to the O2-mediated reaction (e.g., 15- and 3-fold changes for the former and latter reactions, respectively, at pH 8). The steady-state Fe(II) fraction under the simulated sunlight indicated that the Fe(II) fraction varies by up to 12-fold. The correlation analysis indicated that variation of Fe(II) oxidation is significantly associated with aliphatic content of HS, suggesting that Fe(II) complexation by aliphatic components accelerates Fe(II) oxidation. The reduction rate constant and steady-state Fe(II) fractions in the presence of sunlight had relatively strong positive relations with free radical content of HS, possibly due to the reductive property of radical semiquinone in HS. Overall, the findings in this study indicated that the Fe reduction and oxidation kinetics and resultant Fe(II) formation are substantially influenced by chemical properties of HS
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