title: Comparison of different approaches to evaluate External Quality Assessment Data.
abstract: In EQA programs, Z-scores are used to evaluate laboratory performance. They should indicate poorly performing laboratories, regardless of the presence of outliers. For this, two different types of approaches exist. The first type are "outlier-based" approaches, which first exclude outlying values, calculate the average and standard deviation on the remaining data and obtain Z-scores for all values (e.g., Grubbs and Dixon). The second type includes the "robust" approaches (e.g., Tukey and Qn or the algorithm recommended by ISO). The different approaches were assessed by randomly generated samples from the Normal and Student t distributions. Part of the sample data were contaminated with outliers. The number of false and true outliers was recorded and subsequently, Positive and Negative Predictive Values were derived. Also, the sampling mean and variability were calculated for location and scale estimators. The various approaches performed similarly for sample sizes above 10 and when outliers were at good distance from the centre. For smaller sample sizes and closer outliers, however, the approaches performed quite differently. Tukey's method was characterised by a high true and a high false outlier rate, while the ISO and Qn approaches demonstrated weak performance. Grubbs test yielded overall the best results.
authors.affiliation: Scientific Institute of Public Health, Clinical Biology, Brussels, Belgium. wim.coucke@wiv-isp.be
authors.forename: Wim
authors.initials: W
authors.lastname: Coucke
authors.forename: Bernard
authors.initials: B
authors.lastname: China
authors.forename: Isabelle
authors.initials: I
authors.lastname: Delattre
authors.forename: Yolande
authors.initials: Y
authors.lastname: Lenga
authors.forename: Marjan
authors.initials: M
authors.lastname: Van Blerk
authors.forename: Christel
authors.initials: C
authors.lastname: Van Campenhout
authors.forename: Philippe
authors.initials: P
authors.lastname: Van de Walle
authors.forename: Kris
authors.initials: K
authors.lastname: Vernelen
authors.forename: Adelin
authors.initials: A
authors.lastname: Albert
mesh_terms: D002624:Chemistry, Clinical / Q000592:standards*; D019963:Clinical Chemistry Tests / Q000592:standards*; D005069:Evaluation Studies as Topic; D005189:False Positive Reactions; D006801:Humans; D011237:Predictive Value of Tests; D011785:Quality Assurance, Health Care / Q000592:standards*; D018401:Sample Size