Six Sigma quality evaluation of life test data based on Weibull distribution

2020 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Niveditha A ◽  
Ravichandran Joghee
Keyword(s):  
Weibull Distribution ◽  
Test Data ◽  
Six Sigma ◽  
Performance Metrics ◽  
Normal Process ◽  
Life Test ◽  
Content Type ◽  
Life Tests ◽  
Two Parameter ◽  
Theoretical Procedure

PurposeWhile Six Sigma metrics have been studied by researchers in detail for normal distribution-based data, in this paper, we have attempted to study the Six Sigma metrics for two-parameter Weibull distribution that is useful in many life test data analyses.Design/methodology/approachIn the theory of Six Sigma, most of the processes are assumed normal and Six Sigma metrics are determined for such a process of interest. In reliability studies non-normal distributions are more appropriate for life tests. In this paper, a theoretical procedure is developed for determining Six Sigma metrics when the underlying process follows two-parameter Weibull distribution. Numerical evaluations are also considered to study the proposed method.FindingsIn this paper, by matching the probabilities under different normal process-based sigma quality levels (SQLs), we first determined the Six Sigma specification limits (Lower and Upper Six Sigma Limits- LSSL and USSL) for the two-parameter Weibull distribution by setting different values for the shape parameter and the scaling parameter. Then, the lower SQL (LSQL) and upper SQL (USQL) values are obtained for the Weibull distribution with centered and shifted cases. We presented numerical results for Six Sigma metrics of Weibull distribution with different parameter settings. We also simulated a set of 1,000 values from this Weibull distribution for both centered and shifted cases to evaluate the Six Sigma performance metrics. It is found that the SQLs under two-parameter Weibull distribution are slightly lesser than those when the process is assumed normal.Originality/valueThe theoretical approach proposed for determining Six Sigma metrics for Weibull distribution is new to the Six Sigma Quality practitioners who commonly deal with normal process or normal approximation to non-normal processes. The procedure developed here is, in fact, used to first determine LSSL and USSL followed by which LSQL and USQL are obtained. This in turn has helped to compute the Six Sigma metrics such as defects per million opportunities (DPMOs) and the parts that are extremely good per million opportunities (EGPMOs) under two-parameter Weibull distribution for lower-the-better (LTB) and higher-the-better (HTB) quality characteristics. We believe that this approach is quite new to the practitioners, and it is not only useful to the practitioners but will also serve to motivate the researchers to do more work in this field of research.

Six Sigma metrics based on lognormal distribution for life tests

2019 ◽  
Vol 36 (9) ◽  
pp. 1477-1489 ◽  
Author(s):  
Ravichandran Joghee
Keyword(s):  
Six Sigma ◽  
Lognormal Distribution ◽  
Tail Probability ◽  
Tail Probabilities ◽  
Content Type ◽  
Underlying Distribution ◽  
Left Tail ◽  
Long Run ◽  
Life Tests ◽  
The Right

Purpose The purpose of this paper is to propose an approach for studying the Six Sigma metrics when the underlying distribution is lognormal. Design/methodology/approach The Six Sigma metrics are commonly available for normal processes that are run in the long run. However, there are situations in reliability studies where non-normal distributions are more appropriate for life tests. In this paper, Six Sigma metrics are obtained for lognormal distribution. Findings In this paper, unlike the normal process, for lognormal distribution, there are unequal tail probabilities. Hence, the sigma levels are not the same for left-tail and right-tail defects per million opportunities (DPMO). Also, in life tests, while left-tail probability is related to DPMO, the right tail is considered as extremely good PMO. This aspect is introduced and based on which the sigma levels are determined for different parameter settings and left- and right-tail probability combinations. Examples are given to illustrate the proposed approach. Originality/value Though Six Sigma metrics have been developed based on a normality assumption, there have been no studies for determining the Six Sigma metrics for non-normal processes, particularly for life test distributions in reliability studies. The Six Sigma metrics developed here for lognormal distribution is new to the practitioners, and this will motivate the researchers to do more work in this field of research.

scholarly journals Prioritization of Six-Sigma project selection

2016 ◽  
Vol 23 (7) ◽  
pp. 1983-2003 ◽  
Author(s):  
Dotun Adebanjo ◽  
Premaratne Samaranayake ◽  
Fereshteh Mafakheri ◽  
Tritos Laosirihongthong
Keyword(s):  
Six Sigma ◽  
Performance Metrics ◽  
Resource Constraints ◽  
Relative Weight ◽  
Project Selection ◽  
Improvement Project ◽  
Content Type ◽  
Company Directors ◽  
National Organizations ◽  
Research Challenge

Purpose With increasing choice from a range of programs, improvement project selection within broader supply chain context and resource constraints has become a major research challenge. The purpose of this paper is to investigate the different criteria for selecting Six-Sigma (SS) projects based on previous studies. The study is supported by two grounded theories: resource-based view and institutional norms. The criteria include: first, business drivers for improvement and the common performance metrics deployed; second, the organization’s stakeholders needs; and third, process owner’s needs. Design/methodology/approach To determine the relative importance of influencing factors, opinions were collected from 30 experienced practitioners including SS champions/master black-belts, company directors, consultants, and process owners through a series of interviews in small, medium, and large organizations including multi-national organizations. The evaluation of criteria is based on analytical hierarchy process. Findings The results show that impact on customer, financial impacts, and impact on operational goals are the most significant factors in selecting SS improvement project. Originality/value This study is a first attempt to determine the relative weight among SS project selection criteria, which help the practitioner to allocate their limited resources in implementing SS project.

An empirical analysis on Six Sigma implementation in service organisations

2013 ◽  
Vol 4 (2) ◽  
pp. 141-170 ◽  
Author(s):  
Ayon Chakraborty ◽  
Tan Kay Chuan
Keyword(s):  
Six Sigma ◽  
Large Scale ◽  
Success Factors ◽  
Performance Metrics ◽  
Academic Research ◽  
Service Industries ◽  
Service Organizations ◽  
Small Scale ◽  
Widespread Application ◽  
Content Type

PurposeSince its introduction by Motorola in the 1980s, Six Sigma and its philosophy have found widespread application in many manufacturing and service industries. It is felt that there is a need to take stock of the spread of Six Sigma implementation in service organisations. The purpose of this paper is to investigate five main themes which have emerged from literature and to draw on those themes to reflect on wider applicability of Six Sigma in services.Design/methodology/approachThe study involves two exploratory questionnaire surveys. The small‐scale survey is conducted in service organizations in Singapore. The large‐scale survey was web‐based and involved service organisations throughout the world. The objective is to explore and understand the issues highlighted by the service organisations during Six Sigma implementation.FindingsThe findings confirm the inclusion of critical success factors, critical‐to‐quality characteristics, and set of tools and techniques as observed from the literature. In the case of key performance indicators (KPIs), there are different interpretations about them in the literature and also by industry practitioners. Some literature explains KPIs as performance metrics whereas some feel they are key process input or output variables, which is similar to interpretations by practitioners of Six Sigma. The responses of “not relevant” and “unknown to us” as reasons for not implementing Six Sigma show the need for understanding specific requirements of service organizations.Research limitations/implicationsThe limited responses from the authors' surveys restrict the possibility of generalising the findings. Therefore, more extensive survey is required. The three‐phase approach with mixed method used in the overall study has been shown to be useful.Originality/valueAlthough much theoretical description is available about Six Sigma, there has been limited rigorous academic research on it. This gap is far more pronounced about Six Sigma implementation in service organizations, where the theory is not yet mature. Identifying this need, the paper contributes, by empirical research through surveys, to understand the issues involving Six Sigma implementation in service organizations.

Model of the port performance metrics in ports by integration six sigma and system dynamics

2018 ◽  
Vol 35 (1) ◽  
pp. 82-108 ◽  
Author(s):  
Asep Ridwan ◽  
Bernd Noche
Keyword(s):  
Complex System ◽  
System Dynamics ◽  
Six Sigma ◽  
Performance Metrics ◽  
Poor Quality ◽  
Content Type ◽  
Cargo Handling ◽  
Entire Flow ◽  
The Cost ◽  
Metrics Model

Purpose The purpose of this paper is to design a model of the port performance metrics for improving the quality in ports by integration of six sigma and system dynamics (SD) approach. Design/methodology/approach The port performance is measured by the sigma value (SV), the process capability indices (PCIs), and the cost of poor quality (COPQ) as the performance metrics. A port is a complex system that requires SD as an appropriate tool to simulate the model dynamically. The performance metrics focus on measuring the port performance in the entire flow of material in the cargo handling process. Findings With this model, the changing of the SV, the PCIs, and the COPQ can be identified and analyzed the results to improve the performance in ports. These metrics are utilized to eliminate “waste” in the cargo handling process at ports. This waste consists of lost and damaged cargo, equipment and transporter breakdown, and equipment and transporter delay time. The port performance metrics model can assess the causal relationships in ports as a complex system. Originality/value Studies on integration between the six sigma model and SD in ports are few and relatively limited. The port’s performance can be measured directly using the SV, the PCIs, and the COPQ in the simulation. The port performance metrics model will give the decision makers to make some scenarios to contribute for the optimization of performance in ports.

Life Prediction for Linear Rolling Element Bearings: A New Approach to Reliable Life Assessment

2001 ◽  
Vol 124 (1) ◽  
pp. 121-128 ◽  
Author(s):  
Shigeo Shimizu ◽  
Chandra S. Sharma ◽  
Takeki Shirai
Keyword(s):  
Weibull Distribution ◽  
Dynamic Load ◽  
Life Prediction ◽  
Life Assessment ◽  
Rolling Element Bearings ◽  
Life Test ◽  
New Approach ◽  
Rolling Element ◽  
Load Ratings ◽  
Two Parameter

A new approach to the life assessment for linear bearings using three-parameter Weibull distribution is discussed and theoretical basic dynamic load ratings are calculated for linear motion ball guides (LMBG) and ball bushes. Life test experiments are also carried out for these two types of linear bearings with varying design features for each type. The concept of a minimum life prior to which there is no failure, is introduced in the analysis of the life test data. The effects of introducing ball cages in LMBGs and rounding of the edges in the case of ball bushes on their lives, basic dynamic load ratings, and the λbm factors are examined. It is observed that compared with a two-parameter Weibull distribution, an analysis based on a three-parameter Weibull distribution clearly reveals the differences in the lives of linear bearings for changes in their designs.

Evaluating the impact resistance of roller compacted concrete containing crumb rubber and nanosilica using response surface methodology and Weibull distribution

2019 ◽  
Vol 16 (1) ◽  
pp. 33-43 ◽  
Author(s):  
Musa Adamu ◽  
Bashar S. Mohammed ◽  
Mohd Shahir Liew ◽  
Wesam Salah Alaloul
Keyword(s):  
Weibull Distribution ◽  
Impact Resistance ◽  
Crumb Rubber ◽  
Cementitious Materials ◽  
Partial Replacement ◽  
Fine Aggregate ◽  
Content Type ◽  
Roller Compacted Concrete ◽  
The Impact ◽  
Two Parameter

Purpose Roller compacted concrete (RCC) pavement is used in areas subjected to heavy impact loads; therefore, higher impact resistance is a desirable property of consideration. This study aims to investigate the effect of partial replacement of fine aggregate with crumb rubber (CR) and the addition of nanosilica (NS) by weight of cementitious materials on the impact resistance of roller compacted rubbercrete (RCR). Design/methodology/approach Four replacement levels of CR (0, 10, 20 and 30 per cent) and four addition levels of NS (0, 1, 2 and 3 per cent) were considered. The impact resistance test was carried out using the drop weight test recommended by ACI 544. Findings The results showed that the impact resistance of RCR increases with an increase in both CR and NS addition, though for CR above 20 per cent, sudden drop in impact resistance was observed. However, NS reduces the ductility of RCR by decreasing the post-cracking impact resistance. Response surface methodology was used to develop models for predicting the impact resistance of RCR, and the developed models showed a high degree of correlation. As a result of wide variations in the impact drop test data, two-parameter Weibull distribution function was used for the data analysis, and it was found that the probabilistic distributions of the first crack and ultimate failure impact resistance follow the two-parameter Weibull distribution function. Originality/value In this work, the effect of partial replacement of fine aggregate with CR and the addition of NS by weight of cementitious materials on the impact resistance of RCC pavement has been investigated. CR has been used to increase the impact resistance of RCC Pavement.

Maximum-Likelihood Estimation of the Parameters with Constraints in Step-Stress Accelerated Life Tests

2011 ◽  
Vol 291-294 ◽  
pp. 2211-2214
Author(s):  
Yu Hong Xing ◽  
Rui Yuan Liu
Keyword(s):  
Maximum Likelihood ◽  
Maximum Likelihood Estimation ◽  
Test Data ◽  
Likelihood Estimation ◽  
Accelerated Life Test ◽  
Life Test ◽  
Average Lifespan ◽  
Accelerated Life Tests ◽  
Accelerated Life ◽  
Life Tests

This paper investigates the maximum likelihood estimation of the average lifespan of products with the constraints, and the estimation of the average lifespan at stress level, which follows the exponential distribution, is derived by transforming the time-censoring step-stress accelerated life test data into the corresponding constant-stress accelerated life test data. The proposed method can overcome the shortcoming of information lose.

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