title: Much of the variation in breast pathology quality assurance data in the UK can be explained by the random order in which cases arrive at individual centres, but some true outliers do exist.
abstract: OBJECTIVE
  To investigate the role of random temporal order of patient arrival at screening centres in the variability seen in rates of node positivity and breast cancer grade between centres in the NHS Breast Screening Programme.
  
  
  RESULTS
  Computer simulations were performed of the variation in node positivity and breast cancer grade with the random temporal arrival of patients at screening centres based on national UK audit data. Cumulative mean graphs of these data were plotted. Confidence intervals for the parameters were generated, using the binomial distribution. UK audit data were plotted on these control limit graphs. The results showed that much of the variability in the audit data could be accounted for by the effects of random order of arrival of cases at the screening centres. Confidence intervals of 99.7% identified true outliers in the data.
  
  
  CONCLUSIONS
  Much of the variation in breast pathology quality assurance data in the UK can be explained by the random order in which cases arrive at individual centres. Control charts with confidence intervals of 99.7% plotted against the number of reported cases are useful tools for identification of true outliers.
authors: Academic Unit of Pathology, Department of Neuroscience, Faculty of Medicine, Dentistry & Health, University of Sheffield, UK. s.s.cross@sheffield.ac.uk
authors: Simon S
authors: SS
authors: Cross
authors: Timothy J
authors: TJ
authors: Stephenson
authors: Robert F
authors: RF
authors: Harrison
mesh_terms: D001943:Breast Neoplasms / Q000453:epidemiology* / Q000473:pathology; D003198:Computer Simulation; D005260:Female; D006801:Humans; D008207:Lymphatic Metastasis; D060787:Neoplasm Grading; D010338:Pathology, Clinical / Q000592:standards*; D011785:Quality Assurance, Health Care / Q000592:standards*; D006113:United Kingdom