| 1. |
- Albing, Malin, et al.
(author)
-
Elliptical safety region plots for Cpk
- 2011
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In: Journal of Applied Statistics. - : Informa UK Limited. - 0266-4763 .- 1360-0532. ; 38:6, s. 1169-1187
-
Journal article (peer-reviewed)abstract
- The process capability index C pk is widely used when measuring the capability of a manufacturing process. A process is defined to be capable if the capability index exceeds a stated threshold value, e.g. C pk >4/3. This inequality can be expressed graphically using a process capability plot, which is a plot in the plane defined by the process mean and the process standard deviation, showing the region for a capable process. In the process capability plot, a safety region can be plotted to obtain a simple graphical decision rule to assess process capability at a given significance level. We consider safety regions to be used for the index C pk . Under the assumption of normality, we derive elliptical safety regions so that, using a random sample, conclusions about the process capability can be drawn at a given significance level. This simple graphical tool is helpful when trying to understand whether it is the variability, the deviation from target, or both that need to be reduced to improve the capability. Furthermore, using safety regions, several characteristics with different specification limits and different sample sizes can be monitored in the same plot. The proposed graphical decision rule is also investigated with respect to power.
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| 2. |
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| 3. |
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| 4. |
- Albing, Malin, et al.
(author)
-
Skewed zero-bound distributions and process capability indices for upper specifications
- 2009
-
In: Journal of Applied Statistics. - : Informa UK Limited. - 0266-4763 .- 1360-0532. ; 36:2, s. 205-221
-
Journal article (peer-reviewed)abstract
- A common practical situation in process capability analysis, which is not well developed theoretically, is when the quality characteristic of interest has a skewed distribution with a long tail towards relatively large values and an upper specification limit only exists. In such situations it is not uncommon that the smallest possible value of the characteristic is 0 and this also is the best value to obtain. Hence a target value 0 is assumed to exist. We investigate a new class of process capability indices for this situation. Two estimators of the proposed index are studied and the asymptotic distributions of these estimators are derived. Furthermore we suggest a decision procedure useful when drawing conclusions about the capability at a given significance level, based on the estimated indices and their asymptotic distributions. A simulation study is also performed, assuming that the quality characteristic is Weibull distributed, to investigate the true significance level when the sample size is finite.
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| 5. |
- Castagliola, Philippe, et al.
(author)
-
Average run length when monitoring capability indices using EWMA
- 2008
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In: Quality and Reliability Engineering International. - : Wiley. - 0748-8017 .- 1099-1638. ; 24:8, s. 941-955
-
Journal article (peer-reviewed)abstract
- In order to monitor unstable but capable processes Castagliola and Vännman have recently suggested a procedure based on an EWMA approach, called EWMA capability chart, for monitoring Vännman's Cp(u,v)-family of capability indices and showed how their proposed approach efficiently monitors capable processes by detecting a decrease or increase in the capability level. The goal of this paper is to investigate the efficiency of this capability chart in terms of ARL. The procedure used for computing this ARL is presented and simple guidelines for obtaining approximations to the optimal EWMA parameters are proposed.
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| 6. |
- Castagliola, Philippe, et al.
(author)
-
Monitoring capability indices using an EWMA approach
- 2007
-
In: Quality and Reliability Engineering International. - : Wiley. - 0748-8017 .- 1099-1638. ; 23:7, s. 769-790
-
Journal article (peer-reviewed)abstract
- When performing a capability analysis it is recommended to first check that the process is stable, for example, by using control charts. However, there are occasions when a process cannot be stabilized, but it is nevertheless capable. Then the classical control charts fail to efficiently monitor the process position and variability. In this paper we propose a new strategy to solve this problem, where capability indices are monitored in place of the classical sample statistics such as the mean, median, standard deviation, or range. The proposed procedure uses the Cp(u,v) family of capability indices proposed by Vännman combined with a logarithmic transformation and an EWMA approach. One important property of the procedure presented here is that the control limits used for the monitoring of capability indices only depend on the capability level assumed for the process. The experimental results presented in this paper demonstrates how this new approach efficiently monitors capable processes by detecting changes in the capability level.
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| 7. |
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| 8. |
- Castagliola, Philippe, et al.
(author)
-
Monitoring capability indices using run rules
- 2007
-
In: Proceedings of International Conference on Industrial Engineering and Systems Management, ISEM 07. - : ISEM 07. - 9787302153122 - 9787894864390
-
Conference paper (peer-reviewed)
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| 9. |
- Castagliola, Philippe, et al.
(author)
-
Monitoring capability indices using run rules
- 2009
-
In: Journal of Quality in Maintenance Engineering. - : Emerald. - 1355-2511 .- 1758-7832. ; 15:4, s. 358-370
-
Journal article (peer-reviewed)abstract
- PurposeThe purpose of this paper is propose a methodology for monitoring industrial processes that cannot be stabilized, but are nevertheless capable. Design/methodology/approach The proposed procedure uses the C P (u,v) family of capability indices proposed by Vännman (including the indices C PK , C PM , C PMK ) combined with one-sided two-out-of-three and three-out-of-four run rules strategies. FindingsThis paper introduces a new strategy, where capability indices are monitored in place of the classical sample statistics like the mean, median, standard deviation or range. Practical implicationsWhen doing a capability analysis it is recommended to first check that the process is stable, e.g. by using control charts. However, there are occasions when a process cannot be stabilized, but is nevertheless capable. Then the classical control charts fail to efficiently monitor the process position and variability. The approach suggested in this paper overcomes this problem. Originality/valueThe experimental results presented in this paper demonstrate how the new proposed approach efficiently monitors capable processes by detecting decreases or increases of capability level.
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| 10. |
- Castagliola, Philippe, et al.
(author)
-
The efficiency of the EWMA capability chart
- 2007
-
In: IEEE International Conference on Industrial Engineering and Engineering Management, 2007. - Piscataway, NJ : IEEE Communications Society. - 9781424415281 ; , s. 1389-1393
-
Conference paper (peer-reviewed)abstract
- In order to monitor unstable but capable processes Castagliola & Vännman have recently suggested a procedure based on an EWMA approach, called EWMA capability chart, for monitoring Vännman's Cp(u, v)-family of capability indices and showed how their proposed approach efficiently monitors capable processes by detecting a decrease or increase of the capability level. The goal of this paper is to investigate the efficiency of this capability chart by computing optimal EWMA parameters yielding the smallest possible ARL's. The method for computing the ARL is explained and a table summarizing optimal values for the EWMA parameters (λ*,K*) is given for the Cpk chart.
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| 11. |
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| 12. |
- Deleryd, Mats, et al.
(author)
-
Process capability plots—a quality improvement tool
- 1999
-
In: Quality and Reliability Engineering International. - 0748-8017 .- 1099-1638. ; 15:3, s. 213-227
-
Journal article (peer-reviewed)abstract
- We introduce the concept of process capability plots, which are powerful tools to monitor and improve the capability of industrial processes. An advantage of using a process capability plot, compared with using a traditional process capability index alone, when deciding whether a process can be considered capable or not, is that we will instantly get information about the location and spread of the studied characteristic. When the process is non-capable, the plots are helpful when trying to understand if it is the variability, the deviation from target or both that need to be reduced to improve the capability. In this way the proposed graphical methods give a clear direction of quality improvement. We evaluate two different process capability plots, the (δ*, γ*)-plot and the confidence rectangle plot, from a theoretical as well as a practical point of view. When studying them from a theoretical point of view, among other things, a simulation study is conducted to investigate the ability of each of the two methods to identify that a process is capable when it actually is. The comparison from a practical point of view is made by discussing the advantages and disadvantages of the two methods in different practical situations. Based on the above-mentioned comparisons, the recommendation is that the practitioner should use the (δ*, γ*)-plot.
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| 13. |
- Deleryd, Mats, et al.
(author)
-
Process capability studies for short production runs
- 1998
-
In: International Journal of Reliability, Quality and Safety Engineering (IJRQSE). - 0218-5393. ; 5:4, s. 383-401
-
Journal article (peer-reviewed)abstract
- The current trend in modern production is directed towards shorter and shorter production runs. The two major reasons causing this trend are the rapid spread of the just in time (JIT) philosophy and the constantly increasing multiplicity of customer demands. The short runs of modern production not only constitute a challenge for production management, but they also cause some problems when applying traditional statistical methods, designed to be used for large sets of data. One of these methods is process capability studies. Since theories on how to use process capability studies in short production environments are incomplete, the aim of this paper is to present some ideas which will partly fill this gap. The theories of process capability studies for short runs presented are based on ideas of focusing on the process, not on the products, and on using data transformation. By using the transformation presented, it is possible to conduct process capability studies in a traditional straightforward manner. A simulation study shows that the suggested transformation technique works satisfactorily in real situations. Finally, the ....-plot is introduced as a method of interpreting and analyzing the capability of a short run production process. By using the .... -plot, additional information is obtained concerning the capability of a process, compared to using traditional process capability indices only.
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| 14. |
- Hubele, Norma Faris, et al.
(author)
-
The effect of pooled and un-pooled variance estimators on Cpm when using subsamples
- 2004
-
In: Journal of QualityTechnology. - : Informa UK Limited. - 0022-4065. ; 36:2, s. 207-222
-
Journal article (peer-reviewed)abstract
- The vast majority of research on capability indices has assumed that the data consists of one large, representative sample. In practice, and in much of the quality control literature, process data are collected over time in subsamples representing rational subgroups. In this paper we examine the statistical behavior of two Cpm estimators based on this more realistic data structure. The estimators correspond to pooled and un-pooled variance estimators. The theoretical findings are applied to hypothesis testing and power calculations.The power functions of the tests based on the two estimators are used to determine the minimum number of subsamples needed to meet a threshold requirement that power exceeds 0.80. Extensive tables of the recommended number of subsamples are provided with comments on their usage
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| 15. |
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| 16. |
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| 17. |
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| 18. |
- Kvarnström, Björn, et al.
(author)
-
RFID to improve traceability in continuous granular flows : an experimental case study
- 2011
-
In: Quality Engineering. - : Informa UK Limited. - 0898-2112 .- 1532-4222. ; 23:4, s. 343-357
-
Journal article (peer-reviewed)abstract
- Traceability is important for identifying the root-causes of production related quality problems. Traceability can often be reached by adding identification markers on products, but this is not a solution when the value of the individual product is much lower than the incurred cost of a marking system. This is the case for continuous production of granular media. The use of Radio Frequency Identification (RFID) technique to achieve traceability in continuous granular flows has been proposed in the literature. We study through experiments different methods to improve the performance of such an RFID system. For example, larger transponders and multiple readers are shown to improve the RFID system performance.
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| 19. |
- Lundkvist, Peder, et al.
(author)
-
A Comparison of Decision Methods for Cpk When Data are Autocorrelated
- 2012
-
In: Quality Engineering. - : Informa UK Limited. - 0898-2112 .- 1532-4222. ; 24:4, s. 460-472
-
Journal article (peer-reviewed)abstract
- In many industrial applications, autocorrelated data are becoming increasingly common due to, for example, on-line data collection systems with high-frequency sampling. Therefore the basic assumption of independent observations for process capability analysis is not valid. The purpose of this article is to compare decision methods using the process capability index Cpk, when data are autocorrelated. This is done through a case study followed by a simulation study. In the simulation study the actual significance level and power of the decision methods are investigated. The outcome of the article is that two methods appeared to be better than the others.
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| 20. |
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| 21. |
- Tano, Ingrid, 1968-, et al.
(author)
-
Comparing Confidence Intervals for Multivariate Process capability Indices
- 2012
-
In: Quality and Reliability Engineering International. - : Wiley. - 0748-8017 .- 1099-1638. ; 28:4, s. 481-495
-
Journal article (peer-reviewed)abstract
- Multivariate process capability indices (MPCIs) are needed for process capability analysis when the quality of a process is determined by several univariate quality characteristics that are correlated. There are several different MPCIs described in the literature, but confidence intervals have been derived for only a handful of these. In practice, the conclusion about process capability must be drawn from a random sample. Hence, confidence intervals or tests for MPCIs are important. With a case study as a start and under the assumption of multivariate normality, we review and compare four different available methods for calculating confidence intervals of MPCIs that generalize the univariate index Cp. Two of the methods are based on the ratio of a tolerance region to a process region, and two are based on the principal component analysis. For two of the methods, we derive approximate confidence intervals, which are easy to calculate and can be used for moderate sample sizes. We discuss issues that need to be solved before the studied methods can be applied more generally in practice. For instance, three of the methods have approximate confidence levels only, but no investigation has been carried out on how good these approximations are. Furthermore, we highlight the problem with the correspondence between the index value and the probability of nonconformance. We also elucidate a major drawback with the existing MPCIs on the basis of the principal component analysis. Our investigation shows the need for more research to obtain an MPCI with confidence interval such that conclusions about the process capability can be drawn at a known confidence level and that a stated value of the MPCI limits the probability of nonconformance in a known way.
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| 22. |
- Tano, Ingrid, 1968-, et al.
(author)
-
Multivariate process capability analysis applied on a thermal spraying process
- 2011
-
In: 25th International Conference on Surface Modification Technologies, SMT25, University West in Trollhättan, Sweden, on June 20-22, 2011..
-
Conference paper (peer-reviewed)abstract
- AbstractOur interest in multivariate capability indices (MPCI) is based on a case from a thermal spraying process at Volvo Aero Corporation, Sweden, where they wanted to determine the process capability of a three-dimensional variable. The process can be characterized as a high temperature “spray painting” to protect the sprayed surface against, e.g. heat, corrosion and erosion. Different porosity is desirable depending on what the thermally sprayed coating should protect against. There is a relation between the porosity and the in-flame variables and it would be preferable to secure the porosity during spraying instead as of today, afterwards. Since the three-dimensional, in-flame, variables are correlated one way to do this could be to calculate a MPCI of the in-flame variables. There are a number of different MPCIs described in the literature, but for only a handful of these confidence intervals have been derived. In practise the conclusion about process capability must be drawn from a random sample. Hence, confidence or tests for MPCIs are important. Four different available methods for calculating confidence intervals of MPCIs are being reviewed and compared. Current investigation shows issues that need to be solved before the studied methods can be applied more generally in practice.
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| 23. |
- Tano, Ingrid, 1968-, et al.
(author)
-
Multivariate Process Capability Indices—A New Principal Component Analysis Approach
- 2011
-
In: ENBIS1- Coimbra, Portogal.
-
Conference paper (peer-reviewed)abstract
- Often the quality of a process is determined by several correlated quality characteristics. In such cases the quality characteristic should be treated as a vector and a number of different multivariate process capability indices (MPCI:s) have been developed for such a situation. One of the existing MPCIs described in the literature is based on principal component analysis (PCA). The idea behind this MPCI is to do a PCA and consider only the first few principle components that explain the main part of the variability. Then one of the well-known univariate process capability indices is applied to each selected principle component and thereafter the univariate process capability indices for the selected principle components are combined to one MPCI. In order define this MPCI the tolerance region for the quality characteristic vector is transformed to a separate specification interval for each principal component. Recently it was shown that this transformation of the tolerance region into separate specification intervals is done in an improper way. And it is far from obvious how to obtain the individual specification limits for each selected principal component when the transformation is properly made. This problem gets complicated for 2 principal components and even worse for more than 2 principal components. We propose a new method based on PCA that circumvent these difficulties for the case when the tolerance region is a hyper-rectangular. This method first transforms the original data in a suitable way. Then PCA is done on the transformed data and it is shown that only the first principal component is needed to deem a process as capable or not at a stated significance level. Hence, a multivariate situation is transferred into a univariate situation and well-known theory for univariate process capability indices can be used to draw conclusions about the process capability. The properties of this method are investigated through a simulation study.
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| 24. |
- Vanhatalo, Erik, et al.
(author)
-
A designed experiment in a continuous process
- 2007
-
In: Proceedings from the 10th QMOD Conference. - : Lunds University, Campus Helsingborg.
-
Conference paper (peer-reviewed)abstract
- This paper discusses the design and analysis of an experiment performed in a continuous process (CP). A full factorial design with replicates is used to test three types of pellets on two levels of a process variable in an experimental blast furnace process. Issues and considerations concerning the experimental design and analysis are discussed. For example, an adaptive experimental design is used. We propose a multivariate approach to the analysis of the experiment, in form of principal component analysis combined with analysis of variance. The factorial design in CPs is found to have a promising potential. However, CPs also demand special considerations when planning, performing and analyzing experiments, and therefore further development of experimental strategies and connected methods of analysis for CPs is needed.
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| 25. |
- Vanhatalo, Erik, et al.
(author)
-
A method to determine transition time for experiments in dynamic processes
- 2009
-
Conference paper (other academic/artistic)abstract
- Planning, conducting, and analyzing experiments performed in dynamic processes, such as continuous processes, highlight issues that the experimenter needs to consider, for example, process dynamics (inertia) and the multitude of responses. Dynamic systems exhibit a delay (transition time) the change of an experimental factor and when the response is affected. The transition time affects the required length of each experimental run in dynamic processes and long transition times may call for restrictions of the randomization of runs. By contrast, in many processes in parts production this change is almost immediate. Knowledge about the transition time helps the experimenter to avoid experimental runs that are either too short for a new steady-state to be reached, and thus incorrect estimation of treatment effects, or unnecessarily long and costly. Furthermore, knowing the transition time is important during analysis of the experiment.Determining the transition time in a dynamic process can be difficult since the processes often are heavily instrumented with a multitude of responses. The process responses are typically correlated and react to the same underlying events. Hence, multivariate statistical tools such as principal component analysis (PCA) are often beneficial during analysis. Furthermore, the responses are often highly positively autocorrelated due to frequent sampling. We propose a method to determine the transition time between experimental runs in a dynamic process. We use PCA to summarize the systematic variation in a multivariate response space. The time series analysis techniques ‘transfer function-noise modeling' or ‘intervention analysis' are then used to model the dynamic relation between an input time series event and output time series response using the principal component scores. We illustrate the method by estimating the transition time for treatment changes in an experimental blast furnace. This knowledge provides valuable input to the planning and analysis phase of the experiments in the process.
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| 26. |
- Vanhatalo, Erik, et al.
(author)
-
A method to determine transition time for experiments in dynamic processes
- 2011
-
In: Quality Engineering. - : Informa UK Limited. - 0898-2112 .- 1532-4222. ; 23:1, s. 30-45
-
Journal article (peer-reviewed)abstract
- Process dynamics is an important consideration during the planning phase of designed experiments in dynamic processes. After changes of experimental factors, dynamic processes undergo a transition time before reaching a new steady state. To minimize experimental time and reduce costs and for experimental design and analysis, knowledge about this transition time is important. In this article, we propose a method to analyze process dynamics and estimate the transition time by combining principal component analysis and transfer function–noise modeling or intervention analysis. We illustrate the method by estimating transition times for a planned experiment in an experimental blast furnace.
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| 27. |
- Vanhatalo, Erik, et al.
(author)
-
Towards improved analysis methods for two-level factorial experiments with time series responses
- 2013
-
In: Quality and Reliability Engineering International. - : Wiley. - 0748-8017 .- 1099-1638. ; 29:5, s. 725-741
-
Journal article (peer-reviewed)abstract
- Dynamic processes exhibit a time delay between the disturbances and the resulting process response. Therefore, one has to acknowledge process dynamics, such as transition times, when planning and analyzing experiments in dynamic processes. In this article, we explore, discuss, and compare different methods to estimate location effects for two-level factorial experiments where the responses are represented by time series. Particularly, we outline the use of intervention-noise modeling to estimate the effects and to compare this method by using the averages of the response observations in each run as the single response. The comparisons are made by simulated experiments using a dynamic continuous process model. The results show that the effect estimates for the different analysis methods are similar. Using the average of the response in each run, but removing the transition time, is found to be a competitive, robust, and straightforward method, whereas intervention-noise models are found to be more comprehensive, render slightly fewer spurious effects, find more of the active effects for unreplicated experiments and provide the possibility to model effect dynamics. Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd.
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| 28. |
- Vanhatalo, Erik, et al.
(author)
-
Using factorial design and multivariate analysis when experimenting in a continuous process
- 2008
-
In: Quality and Reliability Engineering International. - : Wiley. - 0748-8017 .- 1099-1638. ; 24:8, s. 983-995
-
Journal article (peer-reviewed)abstract
- This article discusses the design and analysis of an experiment performed in a continuous process (CP). Three types of iron ore pellets are tested on two levels of a process variable in an experimental blast furnace process, using a full factorial design with replicates. A multivariate approach to the analysis of the experiment in the form of principal component analysis combined with analysis of variance is proposed. The analysis method also considers the split-plot-like structure of the experiment. The article exemplifies how a factorial design combined with multivariate analysis can be used to perform product development experiments in a CP. CPs also demand special considerations when planning, performing and analyzing experiments. The article highlights and discusses such issues and considerations, for example, the dynamic characteristic of CPs, a strategy to handle disturbances during experimentation and the need for process control during experimentation.
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| 29. |
- Vännman, Kerstin
(author)
-
A general class of capability indices in the case of asymmetric tolerances
- 1997
-
In: Communications in Statistics - Theory and Methods. - : Informa UK Limited. - 0361-0926 .- 1532-415X. ; 26:8, s. 2049-2072
-
Journal article (peer-reviewed)abstract
- Vännman has earlier studied a class of capability indices, containing the indices Cp, Cpk, Cpm, and Cpmk, when the tolerances are symmetric. We study the properties of this class when the tolerances are asymmetric and suggest a new enlargened class of indices. Under the assumption of normality an explicit form of the distribution of the new class of the estimated indices is provided. Numerical investigations are made to explore the behavior of the estimators of the indices for different values of the parameters. Based on the estimator a decision rule that can be used to determine whether the process can be considered capable or not is provided and suitable criteria for choosing an index from the family are suggested.
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| 30. |
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| 31. |
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| 32. |
- Vännman, Kerstin, et al.
(author)
-
A model-free approach to eliminate autocorrelation when testing for process capability
- 2008
-
In: Quality and Reliability Engineering International. - : Wiley. - 0748-8017 .- 1099-1638. ; 24:2, s. 213-228
-
Journal article (peer-reviewed)abstract
- There is an increasing use of on-line data acquisition systems in industry. This usually leads to autocorrelated data and implies that the assumption of independent observations has to be re-examined. Most decision procedures for capability analysis assume independent data. In this article we present a new way of performing capability analysis when data are autocorrelated. This method is based on what can be called the 'iterative skipping' strategy. In that, by skipping a pre-determined number of observations, e.g. considering every fifth observation, the data set is divided into subsamples for which the independence assumption may be valid. For each such subsample of the data we estimate a capability index. Then traditional tests, assuming independence, can be performed based on each estimated capability index from the subsamples. By combining the information from each test statistic based on the subsamples in a suitable way, a new and efficient decision procedure is obtained. We discuss different ways of combining the information from these individual tests. A main appeal of our proposed method is that no time-series model is needed.
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| 33. |
- Vännman, Kerstin, et al.
(author)
-
A statistical approach to composite beams
- 1982
-
In: Nordic Concrete Research, Publication No 1. - : The Nordic Concrete Federation.
-
Conference paper (other academic/artistic)
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| 34. |
- Vännman, Kerstin, et al.
(author)
-
A superstructure of capability indices : distributional properties and implications
- 1995
-
In: Scandinavian Journal of Statistics. - 0303-6898 .- 1467-9469. ; 22:4, s. 477-491
-
Journal article (peer-reviewed)abstract
- A family of capability indices, containing the indices Cp, Cpk, Cpm, and Cpmk, has earlier been defined by {\it K. Vännman} [Stat. Sin. 5, No. 2, 805-820 (1995). for the case of two-sided specification intervals. By varying the parameters of the family various indices with suitable properties can be obtained. Under the assumption of normality an explicit form of the distribution of the family of the estimated indices is provided. Numerical investigations are made to explore the behavior of the estimators of the indices for different values of the parameters. A decision rule, based on the estimator, that can be used to determine whether the process can be considered capable or not is provided and suitable criteria for choosing an index from the family are suggested.
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| 35. |
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| 36. |
- Vännman, Kerstin
(author)
-
A unified approach to capability indices
- 1995
-
In: Statistica sinica. - 1017-0405 .- 1996-8507. ; 5:2, s. 805-820
-
Journal article (peer-reviewed)abstract
- A new class of capability indices, containing Csbp, Csbpk, Csbpm, and Csbpmk, is defined. By varying the parameters of the studied class, indices with different properties can be found. Two estimators of the indices are considered and, assuming that the studied characteristic of the process is normally distributed and that the target value is equal to the midpoint of a two-sided specification interval, their expected values, variances, and mean square errors are derived. It is shown that studying the properties of the class of indices alone, without taking the properties of its estimators into account, might be misleading.
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| 37. |
- Vännman, Kerstin
(author)
-
Att tala med tabeller och diagram
- 1992
-
In: Geometri och statistik. - Lund : Studentlitteratur AB. - 9144354010 ; , s. 155-165
-
Book chapter (pop. science, debate, etc.)
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| 38. |
- Vännman, Kerstin
(author)
-
Att tala med tabeller och diagram?
- 1988
-
In: Svenska matematikersamfundets utbildningsdag -88. ; , s. 31-42
-
Conference paper (other academic/artistic)
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| 39. |
- Vännman, Kerstin
(author)
-
Att utforska data med EDA
- 1984
-
In: Kongressrapport. ; , s. 118-138
-
Conference paper (other academic/artistic)
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| 40. |
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| 41. |
- Vännman, Kerstin
(author)
-
Capability indices when tolerances are asymmetric
- 1996
-
In: Quality improvement through statistical methods. - Boston, Mass : Birkhäuser Verlag. - 0817640525 ; , s. 79-95
-
Conference paper (peer-reviewed)abstract
- Different process capability indices, which have been suggested for asymmetric tolerances, are discussed and, furthermore, two new classes of appropriate indices are suggested. We study the properties of these new classes of indices. We provide, under the assumption of normality, explicit forms of the distributions of these new classes of the estimated indices. We suggest a decision rule to be used to determine if the process can be considered capable. Using the results regarding the distributions it is possible to calculate the probability that the process is to be considered capable. Based on these results we suggest criteria for choosing an index from the class under investigation.
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| 42. |
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| 43. |
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| 44. |
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| 45. |
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| 46. |
- Vännman, Kerstin
(author)
-
Distribution and moments in simplified form for a general class of capability indices
- 1997
-
In: Communications in Statistics - Theory and Methods. - : Informa UK Limited. - 0361-0926 .- 1532-415X. ; 26:1, s. 159-179
-
Journal article (peer-reviewed)abstract
- A class of capability indices, containing the indices Cp, Cpk, Cpm, and Cpmk, has earlier been defined by the author for the case of two-sided specification intervals. By varying the parameters of the class various indices with suitable properties can be obtained. Under the assumption of normality new and simplified expressions of the distribution and the moments of the class of the estimated indices are provided when the target value equals the mid-point of the specification interval. It is shown that the cumulative distribution function can be expressed using only the central χ2-distribution and the normal distribution, which increases the utility of the class of indices.
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| 47. |
- Vännman, Kerstin, et al.
(author)
-
Distributional properties of estimated capability indices based on subsamples
- 2003
-
In: Quality and Reliability Engineering International. - : Wiley. - 0748-8017 .- 1099-1638. ; 19:2, s. 111-128
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Journal article (peer-reviewed)abstract
- Under the assumption of normality, the distribution of estimators of a class of capability indices, containing the indices , , and , is derived when the process parameters are estimated from subsamples. The process mean is estimated using the grand average and the process variance is estimated using the pooled variance from subsamples collected over time for an in-control process. The derived theory is then applied to study the use of hypothesis testing to assess process capability. Numerical investigations are made to explore the effect of the size and number of subsamples on the efficiency of the hypothesis test for some indices in the studied class. The results for and indicate that, even when the total number of sampled observations remains constant, the power of the test decreases as the subsample size decreases. It is shown how the power of the test is dependent not only on the subsample size and the number of subsamples, but also on the relative location of the process mean from the target value. As part of this investigation, a simple form of the cumulative distribution function for the non-central -distribution is also provided
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- Vännman, Kerstin
(author)
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EDA för kreativ statistik
- 1983
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In: Elementa. - 0013-5933. ; :2, s. 59-68
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Journal article (pop. science, debate, etc.)
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| 50. |
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