References of "Zhang, Lixin"
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See detailThe modified Reyment's multivariate coefficient of variation
Zhang, Lixin ULg

Conference (2010, May 18)

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See detailMultivariate extensions of the coefficient of variation with applications
Zhang, Lixin ULg

Conference (2010, March)

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See detailA novel definition of the multivariate coefficient of variation
Albert, Adelin ULg; Zhang, Lixin ULg

in Biometrical Journal = Biometrische Zeitschrift (2010), 52(5), 667-675

The coefficient of variation CV (%) is widely used to measure the relative variation of a random variable to its mean or to assess and compare the performance of analytical techniques/equipments. A review ... [more ▼]

The coefficient of variation CV (%) is widely used to measure the relative variation of a random variable to its mean or to assess and compare the performance of analytical techniques/equipments. A review is made of the existing multivariate extensions of the univariate CV where, instead of a random variable, a random vector is considered, and a novel definition is proposed. The multivariate CV obtained only requires the calculation of the mean vector, the covariance matrix and simple quadratic forms. No matrix inversion is needed which makes the new approach equally attractive inhigh dimensional as in very small sample size problems. As an illustration, the method is applied to electrophoresis data from external quality assessment in laboratory medicine, to phenotypic characteristics of pocket gophers and to a microarray data set. [less ▲]

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See detailThe multivariate coefficient of variation for comparing serum protein electrophoresis techniques in External Quality Assessment schemes
Zhang, Lixin ULg; Albarède, Stéphanie; Dumont, Gilles et al

in Accreditation and Quality Assurance (2010)

External Quality Assessment (EQA) schemes are national or transnational programmes designed to control the analytical performance of clinical laboratories and to maintain inter-laboratory variability ... [more ▼]

External Quality Assessment (EQA) schemes are national or transnational programmes designed to control the analytical performance of clinical laboratories and to maintain inter-laboratory variability within acceptable limits. In such EQA programmes, participants are usually grouped by the type of assay technique/equipment they use. The coefficient of variation (CV) is a simple tool for comparing the inter-laboratory reproducibility of such techniques: the lower the CV, the better the analytical performance. Serum protein electrophoresis, a laboratory test profile consisting of five fractions (albumin, α1, α2, β and γ globulins) summing up to 100% of total proteins, can also be assayed in different ways depending on the media or the analytical principle. We propose a multivariate coefficient of variation for comparing the performance of electrophoretic techniques in EQA, thus extending the univariate CV concept. First, the compositional nature of electrophoretic data requires a one-to-one transformation from the 5-dimensional to the 4-dimensional space. Next, robust estimations of the mean and the covariance matrix are needed to avoid the effect of outliers. The new approach is illustrated on electrophoretic datasets from the French and Belgian national EQA programmes. [less ▲]

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See detailRevisiting the multivariate coefficient of variation for comparing electrophoretic techniques in External Quality Assessment (EQA) schemes
Zhang, Lixin ULg; Albert, Adelin ULg

Conference (2009, October 15)

External Quality Assessment (EQA) schemes are national programmes designed to control the analytical performance of clinical laboratories and to maintain inter-laboratory variability within acceptable ... [more ▼]

External Quality Assessment (EQA) schemes are national programmes designed to control the analytical performance of clinical laboratories and to maintain inter-laboratory variability within acceptable limits. In this context, the type of assay technique or equipment used by the participants is an important factor to take into account. The coefficient of variation (CV) is widely used in EQA to compare the reproducibility of techniques and equipments: the lower the CV, the better the analytical performance. The recent introduction of protein electrophoresis, a laboratory test profile consisting of five fractions (albumin, α1, α2, β and γ globulins) summing up to 100%, in the EQA programme (Zhang et. al. 2008) prompted us to revisit an old problem (Reyment 1960), namely the multivariate extension of the univariate CV concept, and to suggest some amendments to existing solutions. Further, the application of the multivariate CV to techniques and methods used in the assay of protein electrophoresis raises concomitant statistical problems. First, the compositional nature of electrophoretic data requires a log-ratio one-to-one transformation from the simplex to the real space (Aitchison 1982). Next, robust estimations of the mean and the covariance matrix (Rousseeuw and Van Driessen 1999) are needed to avoid the effect of outliers. The resulting robust multivariate CVs are illustrated on electrophoretic EQA material from the Belgian and French national EQA programmes to assess the analytical variability of the electrophoretic techniques used. [less ▲]

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See detailRobust detection of outlying serum protein electrophoretic profiles in External Quality Assessment schemes
Zhang, Lixin ULg

Conference (2008, November 12)

External Quality Assessment (EQA) schemes are national programmes to ensure that results obtained on a particular specimen in a given clinical laboratory are compatible with those obtained by other ... [more ▼]

External Quality Assessment (EQA) schemes are national programmes to ensure that results obtained on a particular specimen in a given clinical laboratory are compatible with those obtained by other laboratories on the same specimen. EQA schemes are traditionally facing the problem of outlier detection of single analytes, e.g., glucose or cholesterol. Recently, serum protein electrophoresis was tested to be subsequently implemented on a regular basis in EQA programmes. This is a laboratory test which consists of five fractions (albumin, α1, α2, β and γ globulins) and the sum of them is equal to 100% of total proteins. So far, in EQA the five fractions have been analyzed separately as for the univariate analytes, without considering the correlation and the linear relationship between them. The minimum covariance determinant (MCD) estimator (Rousseeuw and van Driessen 1999) provides an interesting robust approach to analyze EQA electrophoresis results from a global standpoint. This talk focuses on comparing the results obtained by the traditional univariate approach and the robust MCD approach (Zhang et al. 2008). As illustrated on electrophoretic data from the Belgian EQA scheme, the MCD approach improves the detection of poor performing laboratories. Simple graphical representation of the outliers will be demonstrated at the end of the talk. [less ▲]

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See detailStatistical analysis of serum protein electrophoresis results in External Quality Assessment schemes
Zhang, Lixin ULg; Van Campenhout, Christel; Devleeschouwer, Nicole et al

in Accreditation and Quality Assurance (2008), 13(3), 149-155

The goal of External Quality Assessment (EQA) schemes is to ensure that results obtained on a particular specimen in a given clinical laboratory are compatible with those obtained by other laboratories on ... [more ▼]

The goal of External Quality Assessment (EQA) schemes is to ensure that results obtained on a particular specimen in a given clinical laboratory are compatible with those obtained by other laboratories on the same specimen. Serum protein electrophoresis is a laboratory test consisting of five fractions (albumin, a1, a2, b and c globulins), which sum up to 100% of total proteins. So far, in EQA schemes the five fractions have been analyzed separately as for ordinary tests like glucose or cholesterol. This approach does not consider the fractions as a whole and the linear relationship between them. A statistical approach has been developed to analyze EQA electrophoresis results from a global standpoint by using robust multivariate method to eliminate the effect of outlying profiles. As illustrated on electrophoretic data from the Belgian EQA scheme, the novel approach improves the detection of poor performing laboratories. The method will be implemented in the Belgian EQA scheme on a routine basis. [less ▲]

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See detailComparison between player specialization, anthropometric characteristics and jumping ability in top-level volleybal players
Ciccarone, Guido; Croisier, Jean-Louis ULg; Fontani, G. et al

in Medicina dello Sport : Rivista di Fisiopatologia dello Sport (2008), 61

Aim. Volleyball is an open skill sport with predominant anaerobic alactic acid power. At higher skill levels, performance characteristics are mainly determined by speed and vertical jumping ability. The ... [more ▼]

Aim. Volleyball is an open skill sport with predominant anaerobic alactic acid power. At higher skill levels, performance characteristics are mainly determined by speed and vertical jumping ability. The aim of this study was to compare anthropometric parameters and jumping ability in a group of elite male volleyball players in relation to technical skills required by player position. Methods. The study population included 36 elite male volleyball players (coming from the Italian First and Second Division Championships). Subjects were grouped by playing position on the court: setters (7); blockers (10); hitters (16); liberos (3). Assessment included anthropometric parameters such as height, weight, body mass index (weight in kg/height in m2), indirect calculation of body fat mass by means of the Katch method (%BFM), measurement of reach with one hand (R1) and two hands (R2); and indirect measurement of explosive strength by means of the Bosco method: squat jump, countermovement jump (CMJ), CMJ with arm swing (CMJas), and repeated jumps for 15 s (R15”); measurement of motor coordination and jumping ability by means of the Vertec test combined with specific offense (Vertec attack [VA]) and defense (Vertec block [VB]) skills. Results. Significant differences were observed between player specialization and anthropometric parameters (height, weight, BMI, R1 and R2). Results of the Bosco (SJ, CMJ, CMJas) and Vertec tests (VA and VB) also differed significantly among the player groups. Conclusion. Functional assessment protocols that evaluate anthropometric characteristics and jumping ability by means of two different unrelated methods, confirming that the two measure different aspects of the same performance, provide more complete indications for selecting players, workload planning and monitoring of player development during training. [less ▲]

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See detailStatistical analysis of electrophoresis results in External Quality Assessment schemes
Zhang, Lixin ULg; Libeer, Jean-Claude; Hamers, Nicole et al

Poster (2006, July)

The goal of External Quality Assessment (EQA) schemes is to ensure that results obtained on a particular specimen in a given clinical laboratory do not differ significantly from those obtained by other ... [more ▼]

The goal of External Quality Assessment (EQA) schemes is to ensure that results obtained on a particular specimen in a given clinical laboratory do not differ significantly from those obtained by other laboratories on the same specimen. Hence, EQA aims at improving inter-laboratory agreement. Classically, the same blind control specimen is sent to all laboratories and test results are returned to the EQA organizer and analyzed statistically. Robust methods are used to estimate the mean (target value) and standard deviation (SD, inter-laboratory variability) because of the frequent presence of outliers. Laboratories are considered as “poor performers” if their results depart from the target value by a given threshold (e.g., 3×SD). Serum protein electrophoresis is a laboratory test which yields five fractions (albumin, α1, α2, β and γ) which sum up to 100%. So far, EQA schemes have analyzed the five fractions separately as for ordinary tests. Thus, a laboratory is qualified as a poor performer if at least one fraction is out of range. This approach does not take into account the other fractions and the linear relationship between them. A novel statistical approach has been developed to analyze EQA electrophoresis results from a global standpoint by using multivariate standard and robust methods to eliminate the effect of outlying profiles. When applied to electrophoresis data from the Belgian EQA scheme (n = 189 laboratories), the approach has shown that other laboratories should be considered as poor performers than by the classical univariate method. It has also highlighted the need for taking into account the type of electrophoresis assay in judging the laboratory performance. The method will be implemented routinely in the Belgian EQA scheme. [less ▲]

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See detailpolychlorinated biphenyls contamination in women with breast cancer
Charlier, Corinne ULg; Albert, Adelin ULg; Zhang, Lixin ULg et al

in Clinica Chimica Acta (2004), 347

Detailed reference viewed: 41 (14 ULg)