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  • 51.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Process capability plots: a complement to capability indices2001Ingår 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. 270-283Konferensbidrag (Refereegranskat)
  • 52.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Re-expression - a theme from EDA suitable for the mathematics teacher1992Ingår i: Proceedings of the International Statistical Institute Round Table Conference / [ed] Lionel Pereira-Mendoza, Voorburg: International Statistical Institute , 1992, s. 159-176Konferensbidrag (Refereegranskat)
  • 53.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Safety regions in process capability plots2006Ingår i: Quality Technology & Quantitative Management, ISSN 1684-3703, E-ISSN 1811-4857, Vol. 3, nr 2, s. 227-246Artikel i tidskrift (Refereegranskat)
    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.

  • 54.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Some aspects of statistical graphics for secondary school teachers1990Ingår i: Training Teachers to Teach Statistics: Proceedings of the International Statistical Institute Round Table Conference / [ed] Anne Hawkins, Voorburg: International Statistical Institute , 1990, s. 110-125Konferensbidrag (Refereegranskat)
  • 55.
    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 classroom1995Ingår i: Proceedings of the 50th Session of the International Statistical Institute, 1995, Vol. Book 2, s. 1218-1219Konferensbidrag (Refereegranskat)
  • 56.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Statistics in industry and implications for teaching1986Ingår i: Proceedings: the Second International Conference on Teaching Statistics / [ed] Roger Davidson ; Jim Swift, Victoria, BC: University of Victoria , 1986, s. 355-356Konferensbidrag (Refereegranskat)
  • 57.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Statistics teaching through teacher cooperation and computer in the classroom1997Ingår i: Teorija slucajnych processov, ISSN 0321-3900, Vol. 3(19), nr 1-2, s. 76-87Artikel i tidskrift (Refereegranskat)
    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.

  • 58.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The circular safety region: a useful graphical tool in capability analysis2005Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 21, nr 5, s. 529-538Artikel i tidskrift (Refereegranskat)
    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.

  • 59.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Two teachers, a computer, and statistics: experiences of teacher cooperation1995Ingår i: 8th SEFI European Seminar on Mathematics in Engineering Education: proceedinga, Prague, 1995, Vol. 1, s. 205-211Konferensbidrag (Refereegranskat)
  • 60.
    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 distributions2007Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 23, nr 6, s. 755-765Artikel i tidskrift (Refereegranskat)
    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.

  • 61.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability plots for one-sided specification limits2007Ingår i: Quality Technology & Quantitative Management, ISSN 1684-3703, E-ISSN 1811-4857, Vol. 4, nr 4, s. 569-590Artikel i tidskrift (Refereegranskat)
    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.

  • 62.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability plots for one-sided specification limits2005Rapport (Övrigt vetenskapligt)
  • 63.
    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 Indices2006Ingår 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. 220-224Konferensbidrag (Refereegranskat)
  • 64.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Cederwall, Krister
    Luleå tekniska universitet.
    A statistical approach to composite beams1982Ingår i: Nordic Concrete Research, Publication No 1, The Nordic Concrete Federation , 1982Konferensbidrag (Övrigt vetenskapligt)
  • 65.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Dunkels, Andrejs
    Luleå tekniska universitet.
    Att vara detektiv bland siffror från verkligheten1985Ingår i: Nämnaren : tidskrift för matematikundervisning, ISSN 0348-2723, nr 4, s. 69-Artikel i tidskrift (Övrigt vetenskapligt)
  • 66.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Dunkels, Andrejs
    Luleå tekniska universitet.
    Kreativ statistik med EDA1984Bok (Övrig (populärvetenskap, debatt, mm))
  • 67.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hubele, Norma
    The distribution of Cpm when using a pooled variance estimator with some results2001Ingår i: 2001 proceedings, American Statistical Association: papers presented at the annual meeting of the American Statistical Association: joint statistical meetings, Atlanta, Georgia, August 5 - 9, 2001, and other ASA-sponsored conferences, Acoustical Society of America (ASA), 2001Konferensbidrag (Refereegranskat)
  • 68.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hubele, Norma Faris
    Arizona State University.
    Distributional properties of estimated capability indices based on subsamples2003Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 19, nr 2, s. 111-128Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Under the assumption of normality, the distribution of estimators of a class of capability indices, containing the indices , , and , is derived when the process parameters are estimated from subsamples. The process mean is estimated using the grand average and the process variance is estimated using the pooled variance from subsamples collected over time for an in-control process. The derived theory is then applied to study the use of hypothesis testing to assess process capability. Numerical investigations are made to explore the effect of the size and number of subsamples on the efficiency of the hypothesis test for some indices in the studied class. The results for and indicate that, even when the total number of sampled observations remains constant, the power of the test decreases as the subsample size decreases. It is shown how the power of the test is dependent not only on the subsample size and the number of subsamples, but also on the relative location of the process mean from the target value. As part of this investigation, a simple form of the cumulative distribution function for the non-central -distribution is also provided

  • 69.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hubele, Norma Faris
    The effect of subsample sizes on Cpm when using pooled and un-pooled variance estimators2001Rapport (Övrigt vetenskapligt)
  • 70.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Klefsjö, Bengt
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Ärligare undersökningar tack vare slumpen1983Ingår i: Elementa : tidskrift för matematik, fysik och kemi, ISSN 0013-5933, nr 1, s. 30-34Artikel i tidskrift (Övrig (populärvetenskap, debatt, mm))
  • 71.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kotz, Samuel
    A superstructure of capability indices: asymptotics and its implications1995Ingår i: International Journal of Reliability, Quality and Safety Engineering (IJRQSE), ISSN 0218-5393, Vol. 2, nr 4, s. 343-360Artikel i tidskrift (Refereegranskat)
  • 72.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kotz, Samuel
    University of Maryland.
    A superstructure of capability indices: distributional properties and implications1995Ingår i: Scandinavian Journal of Statistics, ISSN 0303-6898, E-ISSN 1467-9469, Vol. 22, nr 4, s. 477-491Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A family of capability indices, containing the indices Cp, Cpk, Cpm, and Cpmk, has earlier been defined by {\it K. Vännman} [Stat. Sin. 5, No. 2, 805-820 (1995). for the case of two-sided specification intervals. By varying the parameters of the family various indices with suitable properties can be obtained. Under the assumption of normality an explicit form of the distribution of the family of the estimated indices is provided. Numerical investigations are made to explore the behavior of the estimators of the indices for different values of the parameters. A decision rule, based on the estimator, that can be used to determine whether the process can be considered capable or not is provided and suitable criteria for choosing an index from the family are suggested.

  • 73.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kulachi, Murat
    Informatics and Mathematical Modelling, Technical University of Denmark.
    A model free approach to eliminate autocorrelation when testing for process capability2006Rapport (Övrigt vetenskapligt)
  • 74.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kulahci, Murat
    Informatics and Mathematical Modelling, Technical University of Denmark.
    A model-free approach to eliminate autocorrelation when testing for process capability2008Ingår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 24, nr 2, s. 213-228Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    There is an increasing use of on-line data acquisition systems in industry. This usually leads to autocorrelated data and implies that the assumption of independent observations has to be re-examined. Most decision procedures for capability analysis assume independent data. In this article we present a new way of performing capability analysis when data are autocorrelated. This method is based on what can be called the 'iterative skipping' strategy. In that, by skipping a pre-determined number of observations, e.g. considering every fifth observation, the data set is divided into subsamples for which the independence assumption may be valid. For each such subsample of the data we estimate a capability index. Then traditional tests, assuming independence, can be performed based on each estimated capability index from the subsamples. By combining the information from each test statistic based on the subsamples in a suitable way, a new and efficient decision procedure is obtained. We discuss different ways of combining the information from these individual tests. A main appeal of our proposed method is that no time-series model is needed.

12 51 - 74 av 74
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