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  • 1. Albing, Malin
    et al.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Elliptical safety region plots for Cpk2011Ingår i: Journal of Applied Statistics, ISSN 0266-4763, E-ISSN 1360-0532, Vol. 38, nr 6, s. 1169-1187Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The process capability index Cpk 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. Cpk4/3. 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 Cpk. 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

  • 2. Albing, Malin
    et al.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Elliptical Safety Region Plots for Cpk, Cpu and Cpl2008Rapport (Övrigt vetenskapligt)
  • 3. Albing, Malin
    et al.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Process capability indices for one-sided specifications and skew zero-bound distributions2006Rapport (Övrigt vetenskapligt)
  • 4. Albing, Malin
    et al.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Skewed zero-bound distributions and process capability indices for upper specifications2009Ingår i: Journal of Applied Statistics, ISSN 0266-4763, E-ISSN 1360-0532, Vol. 36, nr 2, s. 205-221Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 5. Castagliola, Philippe
    et al.
    Maravelakis, P.
    Psarakis, S.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Monitoring capability indices using run rules2007Ingår i: Proceedings of International Conference on Industrial Engineering and Systems Management, ISEM 07, ISEM 07 , 2007Konferensbidrag (Refereegranskat)
  • 6. Castagliola, Philippe
    et al.
    Maravelakis, P.
    University of the Aegean, Samos.
    Psarakis, S.
    Athens University of Economics and Business.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Monitoring capability indices using run rules2009Ingår i: Journal of Quality in Maintenance Engineering, ISSN 1355-2511, E-ISSN 1758-7832, Vol. 15, nr 4, s. 358-370Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 7. Castagliola, Philippe
    et al.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Average run length when monitoring capability indices using EWMA2008Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 24, nr 8, s. 941-955Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 8.
    Castagliola, Philippe
    et al.
    Université de Nantes and IRCCyN.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Monitoring capability indices using an EWMA approach2007Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 23, nr 7, s. 769-790Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 9.
    Castagliola, Philippe
    et al.
    Université de Nantes.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Monitoring capability indices using EWMA2006Rapport (Övrigt vetenskapligt)
  • 10.
    Castagliola, Philippe
    et al.
    Université de Nantes and IRCCyN.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The efficiency of the EWMA capability chart2007Ingår i: IEEE International Conference on Industrial Engineering and Engineering Management, 2007: IEEM 2007 ; 2 - 4 Dec. 2007, Singapore / [ed] Martin Helander, Piscataway, NJ: IEEE Communications Society, 2007, s. 1389-1393Konferensbidrag (Refereegranskat)
    Abstract [en]

    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.

  • 11.
    Cederwall, Krister
    et al.
    Luleå tekniska universitet.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Composite beams from a statistical point of view1982Rapport (Övrigt vetenskapligt)
  • 12.
    Deleryd, Mats
    et al.
    Luleå tekniska universitet.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Process capability plots: a quality improvement tool1999Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 15, nr 3, s. 213-227Artikel i tidskrift (Refereegranskat)
  • 13.
    Deleryd, Mats
    et al.
    Luleå tekniska universitet.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Process capability studies for short production runs1998Ingår i: International Journal of Reliability, Quality and Safety Engineering (IJRQSE), ISSN 0218-5393, Vol. 5, nr 4, s. 383-401Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 14.
    Hubele, Norma Faris
    et al.
    Arizona State University.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The effect of pooled and un-pooled variance estimators on Cpm when using subsamples2004Ingår i: Journal of QualityTechnology, ISSN 0022-4065, Vol. 36, nr 2, s. 207-222Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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

  • 15.
    Ingri, Johan
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Vattenmonitoringteknik1997Övrigt (Övrig (populärvetenskap, debatt, mm))
  • 16. Kotz, Samuel
    et al.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A superstructure of capability indices: some distributional properties and implications1995Ingår i: Proceedings of the International Conference on Statistical Methods and Statistical Computing for Quality and Productivity Improvement, 1995, Vol. Vol II, s. 822-828Konferensbidrag (Refereegranskat)
  • 17.
    Kulldorff, Gunnar
    et al.
    University of Umeå.
    Vännman, Kerstin
    Estimation of the location and scale parameters of a Pareto distribution by linear functions of order statistics1973Ingår i: Journal of the American Statistical Association, ISSN 0162-1459, E-ISSN 1537-274X, Vol. 68, nr 341, s. 218-227Artikel i tidskrift (Refereegranskat)
  • 18. Kvarnström, Björn
    et al.
    Bergquist, Bjarne
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    RFID to improve traceability in continuous granular flows: an experimental case study2011Ingår i: Quality Engineering, ISSN 0898-2112, E-ISSN 1532-4222, Vol. 23, nr 4, s. 343-357Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 19.
    Lundkvist, Peder
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Vännman, Kerstin
    Kulahci, Murat
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    A Comparison of Decision Methods for Cpk When Data are Autocorrelated2012Ingår i: Quality Engineering, ISSN 0898-2112, E-ISSN 1532-4222, Vol. 24, nr 4, s. 460-472Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 20.
    Tano, Ingrid Andersson
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A multivariate process capability index based on the first principal component only2013Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 29, nr 7, s. 987-1003Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Often the quality of a process is determined by several correlated univariate variables. In such cases, the considered quality characteristic should be treated as a vector. Several different multivariate process capability indices (MPCIs) have been developed for such a situation, but confidence intervals or tests have been derived for only a handful of these. In practice, the conclusion about process capability needs to be drawn from a random sample, making confidence intervals or tests for the MPCIs important. Principal component analysis (PCA) is a well-known tool to use in multivariate situations. We present, under the assumption of multivariate normality, a new MPCI by applying PCA to a set of suitably transformed variables. We also propose a decision procedure, based on a test of this new index, to be used to decide whether a process can be claimed capable or not at a stated significance level. This new MPCI and its accompanying decision procedure avoid drawbacks found for previously published MPCIs with confidence intervals. By transforming the original variables, we need to consider the first principal component only. Hence, a multivariate situation can be converted into a familiar univariate process capability index. Furthermore, the proposed new MPCI has the property that if the index exceeds a given threshold value the probability of non-conformance is bounded by a known value. Properties, like significance level and power, of the proposed decision procedure is evaluated through a simulation study in the two-dimensional case. A comparative simulation study between our new MPCI and an MPCI previously suggested in the literature is also performed. These studies show that our proposed MPCI with accompanying decision procedure has desirable properties and is worth to study further

  • 21.
    Tano, Ingrid Andersson
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Comparing confidence intervals for multivariate process capability indices2012Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 28, nr 4, s. 481-495Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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

  • 22.
    Vanhatalo, Erik
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Bergquist, Bjarne
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Towards improved analysis methods for two-level factorial experiments with time series responses2013Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 29, nr 5, s. 725-741Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 23.
    Vanhatalo, Erik
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Kvarnström, Björn
    Bergquist, Bjarne
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A method to determine transition time for experiments in dynamic processes2009Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    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.

  • 24.
    Vanhatalo, Erik
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Kvarnström, Björn
    Bergquist, Bjarne
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A method to determine transition time for experiments in dynamic processes2011Ingår i: Quality Engineering, ISSN 0898-2112, E-ISSN 1532-4222, Vol. 23, nr 1, s. 30-45Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 25.
    Vanhatalo, Erik
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Using factorial design and multivariate analysis when experimenting in a continuous process2008Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 24, nr 8, s. 983-995Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 26.
    Vanhatalo, Erik
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hyllander, Gunilla
    LKAB, Technology and Business Development.
    A designed experiment in a continuous process2007Ingår i: Proceedings from the 10th QMOD Conference: Our Dreams of Excellence, Lunds University, Campus Helsingborg , 2007Konferensbidrag (Refereegranskat)
    Abstract [en]

    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.

  • 27.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A general class of capability indices in the case of asymmetric tolerances1997Ingår i: Communications in Statistics - Theory and Methods, ISSN 0361-0926, E-ISSN 1532-415X, Vol. 26, nr 8, s. 2049-2072Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 28.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A graphical method to control process capability2001Ingår i: Frontiers in Statistical Quality Control 6: The VIth International Workshop on Intelligent Statistical Quality Control, Würzburg, Germany / [ed] Hans-Joachim Lenz; Peter-Theodor Wilrich, Heidelberg: Physica-Verlag Rudolf Liebig GmbH , 2001, s. 290-311Konferensbidrag (Refereegranskat)
  • 29.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A unified approach to capability indices1995Ingår i: Statistica sinica, ISSN 1017-0405, E-ISSN 1996-8507, Vol. 5, nr 2, s. 805-820Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 30.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A uniformed approach to capability indices1995Ingår i: Statistica sinica, ISSN 1017-0405, E-ISSN 1996-8507, Vol. 5, nr 2, s. 805-820Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A new class of capability indices, containing $C\sb p$, $C\sbpk$, $C\sbpm$, and $C\sbpmk$, 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.

  • 31.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Att tala med tabeller och diagram1992Ingår i: Geometri och statistik, Lund: Studentlitteratur AB, 1992, s. 155-165Kapitel i bok, del av antologi (Övrig (populärvetenskap, debatt, mm))
  • 32.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Att tala med tabeller och diagram?1988Ingår i: Svenska matematikersamfundets utbildningsdag -88: Högskolan i Sundsvall/Härnösand, 1988, s. 31-42Konferensbidrag (Övrigt vetenskapligt)
  • 33.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Att utforska data med EDA1984Ingår i: Kongressrapport: den 12:e nordiske LMFK-kongress, 1984, s. 118-138Konferensbidrag (Övrigt vetenskapligt)
  • 34.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Capability indices when tolerances are asymmetric1996Ingår i: Quality improvement through statistical methods: [...based on the papers presented at the International Conference 'Quality Improvement through Statistical Methods' in Cochin, India during December 28-31, 1996] / [ed] Abraham Bovas, Boston, Mass: Birkhäuser Verlag, 1996, s. 79-95Konferensbidrag (Refereegranskat)
    Abstract [en]

    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.

  • 35.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Come convincere uno studente che uno stimatore é una variabile aleatoria1990Ingår i: Induzioni. Demografia, probabilità, statistica a scuola Nr 0, Centro Interuniversitario di Ricerca per la Didattica delle Discipline Statistiche , 1990, s. 51-58Konferensbidrag (Refereegranskat)
  • 36. Vännman, Kerstin
    Contributions to the theory of statistical inference based on simple functions of order statistics1975Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
  • 37.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Discussant comments on the topic Education and training in statistics for quality improvement and engineering1993Ingår i: Proceedings of the 49th Session of the International Statistical Institute, 1993, Vol. Book 4, s. 53-55Konferensbidrag (Refereegranskat)
  • 38.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Discussion. In Process capability indices, a review, 1992-20002002Ingår i: Journal of QualityTechnology, ISSN 0022-4065, Vol. 34, nr 1, s. 40-42Artikel i tidskrift (Övrigt vetenskapligt)
  • 39.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Distribution and moments in simplified form for a general class of capability indices1997Ingår i: Communications in Statistics - Theory and Methods, ISSN 0361-0926, E-ISSN 1532-415X, Vol. 26, nr 1, s. 159-179Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 40.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    EDA: detektivarbete bland siffror med papper, penna och linjal1980Ingår i: Nämnaren : tidskrift för matematikundervisning, ISSN 0348-2723, nr 4, s. 42-52Artikel i tidskrift (Övrig (populärvetenskap, debatt, mm))
  • 41.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    EDA för kreativ statistik1983Ingår i: Elementa : tidskrift för matematik, fysik och kemi, ISSN 0013-5933, nr 2, s. 59-68Artikel i tidskrift (Övrig (populärvetenskap, debatt, mm))
  • 42.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Encouraging the engineering student to feel the importance of statistics1990Ingår i: Proceedings of the Third International Conference on Teaching Statistics: August 1990, Dunedin, N.Z / [ed] David Vere-Jones, Voorburg: International Statistical Institute , 1990, Vol. 2, s. 245-254Konferensbidrag (Refereegranskat)
  • 43. Vännman, Kerstin
    Estimators based on order statistics from a Pareto distribution1976Ingår i: Journal of the American Statistical Association, ISSN 0162-1459, E-ISSN 1537-274X, Vol. 71, nr 355, s. 704-708Artikel i tidskrift (Refereegranskat)
  • 44.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Families of capability indices for one-sided specification limits1998Ingår i: Statistics (Berlin), ISSN 0233-1888, E-ISSN 1029-4910, Vol. 31, nr 1, s. 43-66Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study, when the specification interval is one-sided, two new classes of indices, which are based on the classes of indices for asymmetric tolerances earlier defined by us in Commun. Stat., Theory Methods 26, 2049-2072 (1997). These classes generalize indices for one-sided specification limits earlier suggested in the literature. By varying the parameters of the classes various indices with suitable properties can be obtained. Under the assumption of normality, explicit forms of the distributions of the two new classes of the estimated indices are provided. Numerical investigations are made to explore the behavior of the estimators of the indices for different values of the parameters. Based on the estimators 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 families are suggested.

  • 45.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Graphical methods to support understanding1991Ingår i: Proceedings of the 48th Session of the International Statistical Institute, 1991Konferensbidrag (Refereegranskat)
  • 46.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    How to Convince a Student that an Estimator is a Random Variable1983Ingår i: Teaching Statistics, ISSN 0141-982X, E-ISSN 1467-9639, Vol. 5, nr 2, s. 49-54Artikel i tidskrift (Refereegranskat)
  • 47.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Matematisk statistik2002Bok (Övrig (populärvetenskap, debatt, mm))
  • 48.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Med en god start på lektionen kan man skatta sig lycklig1983Ingår i: Rapport från Svenska statistikersamfundets utbildningsdag: 21 oktober 1983, 1983, s. 31-37Konferensbidrag (Övrigt vetenskapligt)
  • 49.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On the distribution of the estimated mean from nonstandard mixtures of distributions1995Ingår i: Communications in Statistics - Theory and Methods, ISSN 0361-0926, E-ISSN 1532-415X, Vol. 24, nr 6, s. 1569-1584Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The distribution of the estimated mean of the nonstandard mixture of distributions that has a discrete probability mass at zero and a gamma distribution for positive values is derived. Furthermore, for the studied nonstandard mixture of distributions, the distribution of the standardized statistic (estimator - true mean)/standard deviation of estimator is derived. The results are used to study the accuracy of the confidence interval for the mean based on a large sample approximation. Quantiles for the standardized statistic are also calculated.

  • 50.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Process capability plots2007Ingår i: Encyclopedia of Statistics in Quality and Reliability, Chichester: John Wiley & Sons, 2007, s. 1470-75Kapitel i bok, del av antologi (Övrigt vetenskapligt)
    Abstract [en]

    A process is usually defined to be capable if a process capability index ex­ceeds a stated threshold value. This definition can be expressed graphi­cally as a region in the plane, defined by the process parameters, to obtain a process capability plot. Under the assumption of normality, safety re­gions can be plotted in the process capa­bility plot to draw conclusions about process capa­bility at a given signi­ficance level. Alterna­tive­ly, an estimated process capability plot defined by the estimators of process para­meters can be used. This article discusses these different kinds of plots and presents examples and interpretations.

12 1 - 50 av 74
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