title: Effect of variability in assay bias and imprecision on external quality assessment bias and imprecision measures.
abstract: BACKGROUND
  External quality assessment/proficiency testing programmes that report consensus means and coefficients of variations (CVs) are a potential source of information on assay bias and imprecision. This study examined the effect of variability in assay bias and imprecision on consensus means and CVs, using a computerized spreadsheet model.
  
  
  METHODS
  A model with varying assay bias (mean and standard deviation) and assay imprecision (mean and standard deviation) was developed using a MS Excel 2003 spreadsheet with a macro to generate pseudo-random numbers from a Gaussian distribution. The means, standard deviations and CVs of these data points were considered to simulate the consensus measures reported in external quality assessment testing programmes.
  
  
  RESULTS
  The simulated consensus CV was very sensitive to increases in assay bias variability, with greater effects at low levels of mean assay imprecision. Changes in assay imprecision variability had a much smaller effect on the simulated consensus CV, while the simulated consensus mean was relatively insensitive to any change in the other variables.
  
  
  CONCLUSIONS
  Consensus imprecision measures from external quality assessment programmes are unreliable measures of assay imprecision. Consensus means are relatively unaffected by variability in assay bias and imprecision, and can be used as reliable measures of assay bias from a statistical standpoint.
authors.affiliation: Department of Pathology and Laboratory Medicine, Tan Tock Seng Hospital, Singapore 308433. Robert_Hawkins@ttsh.com.sg
authors.forename: Robert C
authors.initials: RC
authors.lastname: Hawkins
mesh_terms: D015982:Bias; D003198:Computer Simulation; D015233:Models, Statistical