title: A modified statistical approach for the detection of outlying values in external quality control: comparison with other techniques.
abstract: Laboratory results submitted to External Quality Assessment Schemes (EQAS) are evaluated against the arithmetic mean (m) and standard deviation (SD) of the result from other participants assuming a normal distribution of individual results. The sample statistics m and SD can only be reliable estimates of the theoretical mean (mu) and standard deviation (sigma), if they are not distorted by outlying values. Outliers are commonly detected by defining a confidence interval m +/- k SD, typically with k-values fixed at 3.0 or 3.2. The objective of our study was to prove the deficiency of the use of these fixed limits in large-scale EQAS, because it results in an unacceptable increase of outlier rate and hence in a serious distortion of the estimate of sigma. We have proposed a modified approach using a variable k-value which is dependent upon sample size and keeping the significance level constant at 5%. The influence of different decision limits on the number of outliers and on the sample statistics m and SD was compared using data from the Belgian National EQAS obtained in 6 consecutive surveys.
authors: Laboratoria voor Medische Biochemie en voor Klinische Analyse, Rijksuniversiteit Gent (RUG), Belgium.
authors: L M
authors: LM
authors: Thienpont
authors: H L
authors: HL
authors: Steyaert
authors: A P
authors: AP
authors: De Leenheer
mesh_terms: D001774:Blood Chemical Analysis; D001786:Blood Glucose / Q000032:analysis; D006801:Humans; D006854:Hydrocortisone / Q000097:blood; D007501:Iron / Q000097:blood; D007753:Laboratories / Q000592:standards*; D011336:Probability; D011786:Quality Control
chemical_list: D001786:Blood Glucose; D007501:Iron; D006854:Hydrocortisone