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  • 51.
    Migdalas, Athanasios
    et al.
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Zioutas, George
    Chatzinakos, Chris
    New Statistical Robust Estimators: Open Problems2018Inngår i: Open Problems in Optimization and Data Analysis / [ed] Pardalos, Panos M., Springer Publishing Company, 2018, s. 23-47Kapittel i bok, del av antologi (Fagfellevurdert)
    Abstract [en]

    Computational and theoretical open problems in optimization, computational geometry, data science, logistics, statistics, supply chain modeling, and data analysis are examined in this book.  Each contribution provides the fundamentals  needed to fully comprehend the impact of individual problems. Current theoretical, algorithmic, and practical methods used to circumvent each problem are provided to stimulate a new effort towards innovative and efficient solutions. Aimed towards graduate students and researchers in mathematics, optimization, operations research, quantitative logistics, data analysis, and statistics, this book provides a broad comprehensive approach to understanding the significance of specific challenging or open problems within each discipline.

  • 52.
    Mishra, Madhav
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
    Martinsson, Jesper
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rantatalo, Matti
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
    Goebel, Kai
    NASA Ames Research Center, Intelligent Systems Division, Moffett Field, CA.
    Bayesian hierarchical model-based prognostics for lithium-ion batteries2018Inngår i: Reliability Engineering & System Safety, ISSN 0951-8320, E-ISSN 1879-0836, Vol. 172, s. 25-35Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    To optimise operation and maintenance, knowledge of the ability to perform the required functions is vital. The ability is governed by the usage of the system (operational issues) and availability aspects like reliability of different components. This paper proposes a Bayesian hierarchical model (BHM)-based prognostics approach applied to Li-ion batteries, where the goal is to analyse and predict the discharge behaviour of such batteries with variable load profiles and variable amounts of available discharge data. The BHM approach enables inferences for both individual batteries and groups of batteries. Estimates of the hierarchical model parameters and the individual battery parameters are presented, and dependencies on load cycles are inferred. A BHM approach where the operational and reliability aspects end of life (EoD) and end of life (EoL) is studied where its shown that predictions of EoD can be made accurately with a variable amount of battery data. Without access to measurements, e.g. predicting a new battery, the predictions are based only on the prior distributions describing the similarity within the group of batteries and their dependency on the load cycle. A discharge cycle dependency can also be identified in the result giving the opportunity to predict the battery reliability.

  • 53.
    Mishra, Madhav
    et al.
    Division of Operation and Maintenance Engineering within the framework of the SKF-University of Technology Centre (UTC).
    Martinsson, Jesper
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper. Luossavaara-Kiirunavaara AB.
    Rantatalo, Matti
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
    Goebel, Kai
    NASA Ames Research Center, Intelligent Systems Division, Moffett Field, CA, USA.
    Hierarchical model-based prognostics for Li-ion batteries2018Inngår i: Advances In ENGINEERINGArtikkel i tidsskrift (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    Recent global trend towards a fossil-fuel-free society has yielded the rapid soar in demand of electrically powered systems. Specifically, the demand for battery powered systems has fueled the desire to have better performing batteries with lithium-ion batteries being the most widely used. Presently, for application in unmanned vehicles, exploratory rovers, submarines among others demand a better comprehension of battery performance metrics. Case and point, battery capacity and state of charge have become increasingly vital when it comes to determining the end of discharge. As of now, several techniques have already been established for determining such parameters. Unfortunately, their prognostic capability for determining remaining battery charge is still not optimal. Therefore, there is a need to develop prognostic and health management technology for critical systems (such as Mars rovers) to successfully predict and manage the lifetime of batteries, monitor their health state in real time, evaluate the performance and predict the remaining useful life.

    To this note, Luleå University of Technology researchers in Sweden: Dr. Madhav Mishra, Dr. Jesper Martinsson, and Dr. Matti Rantatalo in collaboration with Dr. Kai Goebel at NASA in the United States proposed a study whose main objective was to measure the battery discharge and predict the end of discharge considering the operating conditions for lithium ion batteries. To be precise, they purposed on employing a Bayesian Hierarchical Model (BHM)-based end of discharge prognostic for Li-ion batteries. Their work is currently published in the research journal, Reliability Engineering and System Safety.

    The research technique employed entailed the utilization of two batteries with 16 discharge events with a simplified battery circuit model of the battery. Next, the research team examined the detailed discharge voltage profiles during different discharging cycles with variable load profiles. They then proceeded to demonstrate the BHM approach and group-level dependencies by utilizing more than one battery and more than one discharge cycle.

    The authors observed that the BHM approach enabled inferences for both individual batteries and groups of batteries. The researchers then recorded the estimates of the hierarchical model parameters and the individual battery parameters after which their dependencies on load cycles were inferred. In addition, they noted that by using the BHM approach the predictions of end of discharge could be made accurately with a variable amount of battery data. Furthermore, this technique was seen to applicable even for new batteries without prior recorded data where the predictions were based only on the prior distributions describing the similarity within the group of batteries and their dependency on the load cycle.

    In conclusion, the study presented a Bayesian hierarchical model (BHM)-based prognostics approach for Li-ion batteries, where the goal was to analyze and predict the discharge behavior of such batteries with variable load profiles and variable amounts of available discharge data. The results obtained showed that the technique could address cases with or without data. Altogether, the proposed method can capture additional relationships between parameters and use it to improve prognostics. Lastly, the BHM approach has been seen to permit inference at both the individual battery level and group of battery level.

  • 54. Silvestrov, D.S.
    Exponential asymptotics for a perturbed renewal equation1996Inngår i: Theory of Probability and Mathematical Statistics, ISSN 0094-9000, nr 52, s. 153-162Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Exponential asymptotic expansions of a new type are obtained for solutions of perturbed renewal equations. Some important ergodic theorems and limit theorems on the convergence in law of stopping time functionals for regenerative processes are generalized for mixed versions of these theorems. These theorems applied to stopping times cover both theorems on the convergence of distributions and large-deviation-type theorems

  • 55.
    Tano, Ingrid
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Contributions to multivariate process capability indices2012Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The work presented in this thesis considers multivariate process capability indices (MPCIs) with focus on confidence intervals and tests for MPCIs. It also includes a case study, where multivariate statistical analysis and MPCIs are applied to data from a thermal spraying process at Volvo Aero Corporation. A process capability index measures the ability of a process to satisfy customers’ demands. Since the knowledge about the process performance is based on a random sample, it is important to be able to handle the uncertainty this implies. If the uncertainty is not dealt with and the estimated index is used directly without considering, e.g. a confidence interval, an erroneous conclusion may be drawn about the process capability.The thesis consists of a summary and of three papers, one of which has been presented at an international conference, one has been accepted for publication in an international journal, and the third has been submitted for publication.In Paper I, multivariate statistical analysis is used for screening and tentative model building to describe the relationship between the porosity and the heat conductivity in the thermal spraying process. Object-oriented finite element analysis (OOF) is subsequently used for verification of the statistical model. The new approach functions well and confirms the findings from the statistical model.Paper II reviews and compares four different MPCIs with confidence intervals, requiring multivariate normal distribution. MPCIs are needed when the quality characteristic of interest is multivariate, i.e. when the quality characteristic consists of several correlated variables. The review shows that more research is needed to obtain an MPCI with a confidence interval or a test that works properly. In particular, it is required that a stated value of the MPCI at least limits the probability of non-conformance in a known way. A drawback is also elucidated with existing MPCIs based on principal component analysis.Paper III presents some method development with the purpose of meeting the deficiencies identified in Paper II. A new index together with a confidence interval and a decision procedure is developed that converts a multivariate situation into a familiar univariate situation. Well-known statistical theory for univariate process capability indices can then be used. Properties, like significance level and power, of the pro¬posed deci¬sion procedure is evaluated through a simulation study in the two-dimen¬sional case. A comparative simulation study between our new MPCI and an MPCI pre¬viously suggested in the literature is also done. These studies show that our proposed MPCI with accompanying decision procedure has desirable properties and is worth to study further.

  • 56.
    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 only2013Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 29, nr 7, s. 987-1003Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 57.
    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 indices2012Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 28, nr 4, s. 481-495Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 58.
    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 responses2013Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 29, nr 5, s. 725-741Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 59.
    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 processes2009Konferansepaper (Annet vitenskapelig)
    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.

  • 60.
    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 processes2011Inngår i: Quality Engineering, ISSN 0898-2112, E-ISSN 1532-4222, Vol. 23, nr 1, s. 30-45Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 61.
    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 process2008Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 24, nr 8, s. 983-995Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 62.
    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 process2007Inngår i: Proceedings from the 10th QMOD Conference: Our Dreams of Excellence, Lunds University, Campus Helsingborg , 2007Konferansepaper (Fagfellevurdert)
    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.

  • 63.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A general class of capability indices in the case of asymmetric tolerances1997Inngår i: Communications in Statistics - Theory and Methods, ISSN 0361-0926, E-ISSN 1532-415X, Vol. 26, nr 8, s. 2049-2072Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 64.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A graphical method to control process capability2001Inngår 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. 290-311Konferansepaper (Fagfellevurdert)
  • 65.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A unified approach to capability indices1995Inngår i: Statistica sinica, ISSN 1017-0405, E-ISSN 1996-8507, Vol. 5, nr 2, s. 805-820Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 66.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A uniformed approach to capability indices1995Inngår i: Statistica sinica, ISSN 1017-0405, E-ISSN 1996-8507, Vol. 5, nr 2, s. 805-820Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 67.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Att tala med tabeller och diagram1992Inngår i: Geometri och statistik, Lund: Studentlitteratur AB, 1992, s. 155-165Kapittel i bok, del av antologi (Annet (populærvitenskap, debatt, mm))
  • 68.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Att tala med tabeller och diagram?1988Inngår i: Svenska matematikersamfundets utbildningsdag -88: Högskolan i Sundsvall/Härnösand, 1988, s. 31-42Konferansepaper (Annet vitenskapelig)
  • 69.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Att utforska data med EDA1984Inngår i: Kongressrapport: den 12:e nordiske LMFK-kongress, 1984, s. 118-138Konferansepaper (Annet vitenskapelig)
  • 70.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Capability indices when tolerances are asymmetric1996Inngår 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. 79-95Konferansepaper (Fagfellevurdert)
    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.

  • 71.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Come convincere uno studente che uno stimatore é una variabile aleatoria1990Inngår i: Induzioni. Demografia, probabilità, statistica a scuola Nr 0, Centro Interuniversitario di Ricerca per la Didattica delle Discipline Statistiche , 1990, s. 51-58Konferansepaper (Fagfellevurdert)
  • 72. Vännman, Kerstin
    Contributions to the theory of statistical inference based on simple functions of order statistics1975Doktoravhandling, med artikler (Annet vitenskapelig)
  • 73.
    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 engineering1993Inngår i: Proceedings of the 49th Session of the International Statistical Institute, 1993, Vol. Book 4, s. 53-55Konferansepaper (Fagfellevurdert)
  • 74.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Discussion. In Process capability indices, a review, 1992-20002002Inngår i: Journal of QualityTechnology, ISSN 0022-4065, Vol. 34, nr 1, s. 40-42Artikkel i tidsskrift (Annet vitenskapelig)
  • 75.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Distribution and moments in simplified form for a general class of capability indices1997Inngår i: Communications in Statistics - Theory and Methods, ISSN 0361-0926, E-ISSN 1532-415X, Vol. 26, nr 1, s. 159-179Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 76.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    EDA: detektivarbete bland siffror med papper, penna och linjal1980Inngår i: Nämnaren : tidskrift för matematikundervisning, ISSN 0348-2723, nr 4, s. 42-52Artikkel i tidsskrift (Annet (populærvitenskap, debatt, mm))
  • 77.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    EDA för kreativ statistik1983Inngår i: Elementa : tidskrift för matematik, fysik och kemi, ISSN 0013-5933, nr 2, s. 59-68Artikkel i tidsskrift (Annet (populærvitenskap, debatt, mm))
  • 78.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Encouraging the engineering student to feel the importance of statistics1990Inngår 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. 245-254Konferansepaper (Fagfellevurdert)
  • 79. Vännman, Kerstin
    Estimators based on order statistics from a Pareto distribution1976Inngår i: Journal of the American Statistical Association, ISSN 0162-1459, E-ISSN 1537-274X, Vol. 71, nr 355, s. 704-708Artikkel i tidsskrift (Fagfellevurdert)
  • 80.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Families of capability indices for one-sided specification limits1998Inngår i: Statistics (Berlin), ISSN 0233-1888, E-ISSN 1029-4910, Vol. 31, nr 1, s. 43-66Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 81.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Graphical methods to support understanding1991Inngår i: Proceedings of the 48th Session of the International Statistical Institute, 1991Konferansepaper (Fagfellevurdert)
  • 82.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    How to Convince a Student that an Estimator is a Random Variable1983Inngår i: Teaching Statistics, ISSN 0141-982X, E-ISSN 1467-9639, Vol. 5, nr 2, s. 49-54Artikkel i tidsskrift (Fagfellevurdert)
  • 83.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Matematisk statistik2002Bok (Annet (populærvitenskap, debatt, mm))
  • 84.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Med en god start på lektionen kan man skatta sig lycklig1983Inngår i: Rapport från Svenska statistikersamfundets utbildningsdag: 21 oktober 1983, 1983, s. 31-37Konferansepaper (Annet vitenskapelig)
  • 85.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On the distribution of the estimated mean from nonstandard mixtures of distributions1995Inngår i: Communications in Statistics - Theory and Methods, ISSN 0361-0926, E-ISSN 1532-415X, Vol. 24, nr 6, s. 1569-1584Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 86.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Process capability plots2007Inngår i: Encyclopedia of Statistics in Quality and Reliability, Chichester: John Wiley & Sons, 2007, s. 1470-75Kapittel i bok, del av antologi (Annet vitenskapelig)
    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.

  • 87.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Process capability plots: a complement to capability indices2001Inngår 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. 270-283Konferansepaper (Fagfellevurdert)
  • 88.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Re-expression - a theme from EDA suitable for the mathematics teacher1992Inngår i: Proceedings of the International Statistical Institute Round Table Conference / [ed] Lionel Pereira-Mendoza, Voorburg: International Statistical Institute , 1992, s. 159-176Konferansepaper (Fagfellevurdert)
  • 89.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Safety regions in process capability plots2006Inngår i: Quality Technology & Quantitative Management, ISSN 1684-3703, E-ISSN 1811-4857, Vol. 3, nr 2, s. 227-246Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A process is usually defined to be capable if the process capability index exceeds a stated threshold value, e.g., Cpm > 4/3. This inequality can be expressed graphically as a region in the plane defined by the process parameters (,μσ). In the obtained plot special regions can be plotted to test for process capability. These regions are similar to confidence regions for (,μσ). This idea of using regions in process capability plots to assess the capability is developed further for the capability index Cpm. A new circular region is constructed that can be used, in a simple graphical way, to draw conclusions about the capability of the process at a given significance level. Using circular regions several characteristics with different specification limits and different sample sizes can be monitored in the same plot. Under the assumption of normality the suggested method is investigated with respect to power as well as compared to other existing graphical methods for drawing inference about process capability.

  • 90.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Some aspects of statistical graphics for secondary school teachers1990Inngår i: Training Teachers to Teach Statistics: Proceedings of the International Statistical Institute Round Table Conference / [ed] Anne Hawkins, Voorburg: International Statistical Institute , 1990, s. 110-125Konferansepaper (Fagfellevurdert)
  • 91.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Some experiences of teacher cooperation involving statistics, geochemistry, and a computer in the classroom1995Inngår i: Proceedings of the 50th Session of the International Statistical Institute, 1995, Vol. Book 2, s. 1218-1219Konferansepaper (Fagfellevurdert)
  • 92.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Statistics in industry and implications for teaching1986Inngår i: Proceedings: the Second International Conference on Teaching Statistics / [ed] Roger Davidson ; Jim Swift, Victoria, BC: University of Victoria , 1986, s. 355-356Konferansepaper (Fagfellevurdert)
  • 93.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Statistics teaching through teacher cooperation and computer in the classroom1997Inngår i: Teorija slucajnych processov, ISSN 0321-3900, Vol. 3(19), nr 1-2, s. 76-87Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The author describes his experience in developing the course of statistical analysis applications in geochemistry. During this course the students study graphical statistical methods, exploratory data analysis, smoothing and outlier treatment, comparison of two and more samples and regression analysis including residual analysis and influence analysis. The geochemistry part of the course includes monitoring techniques for water quality data and normalising of hydrogeochemical data. The analysis was performed by Data Desk program. The specific feature of the course was that two teachers one from geochemistry and one from statistics were active simultaneously in the classroom during part of the course. They participated in discussions on the interpretation of results. Students were actively involved in the discussions and statistical analysis.

  • 94.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The circular safety region: a useful graphical tool in capability analysis2005Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 21, nr 5, s. 529-538Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When measuring the capability of a manufacturing process some form of process capability index is often used. To assess the capability, using a random sample, confidence intervals or hypothesis tests on the index are frequently used. Here, an alternative approach is presented. A process is usually defined to be capable if the capability index exceeds a stated threshold value, e.g. Cpm > 4/3. This inequality can be expressed graphically as a region in the plane defined by the process parameters (µ, σ). This graphical region, or safety region, similar to a confidence region for ((µ, σ). can be plotted to test for capability. Under the assumption of normality a circular safety region for the capability index Cpm is constructed that can be used to draw conclusions about the capability at a given significance level. This simple graphical approach is 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. Using circular regions several characteristics can be monitored in the same plot.

  • 95.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Two teachers, a computer, and statistics: experiences of teacher cooperation1995Inngår i: 8th SEFI European Seminar on Mathematics in Engineering Education: proceedinga, Prague, 1995, Vol. 1, s. 205-211Konferansepaper (Fagfellevurdert)
  • 96.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability indices for one-sided specification intervals and skewed distributions2007Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 23, nr 6, s. 755-765Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    One-sided specification intervals are frequent in industry, but the process capability analysis is not well developed theoretically for this case. Most of the published articles about process capability focus on the case when the specification interval is two-sided. Furthermore, usually the assumption of normality is necessary. However, a common practical situation is process capability analysis when the studied characteristic has a skewed distribution with a long tail towards 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 that this also is the best value to obtain. We propose a new class of indices for such a situation with an upper specification limit, a target value zero, and where the studied characteristic has a skewed, zero-bound distribution with a long tail towards large values. A confidence interval for an index in the proposed class, as well as a decision procedure for deeming a process as capable or not, is discussed. These results are based on large sample properties of the distribution of a suggested estimator of the index. A simulation study is performed, assuming the quality characteristic is Weibull distributed, to investigate the properties of the suggested decision procedure.

  • 97.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability plots for one-sided specification limits2007Inngår i: Quality Technology & Quantitative Management, ISSN 1684-3703, E-ISSN 1811-4857, Vol. 4, nr 4, s. 569-590Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We extend the idea of process capability plots from the case of two-sided specification intervals to derive a graphical method useful when doing capability analysis having one-sided specification limits. The derived process capability plots are based on existing capability indices for one-sided specification limits. Both the cases with and without a target value are investigated. Under the assumption of normality we suggest estimated process capability plots to be used to assess process capability at a given significance level. Theoretical results are given for determining the significance level as well as power for the method. The presented graphical approach is helpful to determine if it is the variability, the deviation from target, or both that need to be reduced to improve the capability. Examples are presented.

  • 98.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability plots for one-sided specification limits2005Rapport (Annet vitenskapelig)
  • 99.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Castagliola, Philippe
    Monitoring Unstable But Capable Processes with the CP(u,v) System of Capability Indices2006Inngår i: 2006 proceedings: Twelfth ISSAT International Conference Reliability and Quality in Design : August 3 - 5, 2006, Chicago, Illinois, U.S.A. / [ed] Hoang Pham, New Brunswick, NJ: International Society of Science and Applied Technologies , 2006, s. 220-224Konferansepaper (Fagfellevurdert)
  • 100.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Cederwall, Krister
    Luleå tekniska universitet.
    A statistical approach to composite beams1982Inngår i: Nordic Concrete Research, Publication No 1, The Nordic Concrete Federation , 1982Konferansepaper (Annet vitenskapelig)
123 51 - 100 of 111
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