title: Agreement or prediction: asking and answering the right question.
abstract: BACKGROUND
  Measuring agreement and measuring predictive ability are similar but distinct problems. Failure to appreciate the conceptual and practical differences may lead clinical researchers to give the right answer to the wrong question.
  
  
  METHODS
  We illustrate the relation and difference between measuring agreement and predictive ability in a non-technical way. We provide a real example investigating the feasibility of using preoperative breast cancer tumour size measurements to estimate postoperative histological size. The intraclass correlation and R-squared are calculated to ascertain the level of agreement and predictive ability respectively.
  
  
  RESULTS
  Analysis of agreement and analysis of predictive ability serve different purposes. The optimal solution found in terms of agreement may be different from that found for prediction.
  
  
  CONCLUSIONS
  A careful clarification of the goal of an investigation is important. Using an inappropriate analysis can lead to misleading results, or to results that do not really answer the research question of interest.
authors.affiliation: Department of Surgical Oncology, National Cancer Centre, 11 Hospital Drive, Singapore 169610.
authors.forename: S B
authors.initials: SB
authors.lastname: Tan
authors.forename: S B
authors.initials: SB
authors.lastname: Wee
authors.forename: Y B
authors.initials: YB
authors.lastname: Cheung
mesh_terms: D000886:Anthropometry / Q000379:methods; D001943:Breast Neoplasms / Q000473:pathology / Q000601:surgery; D003627:Data Interpretation, Statistical; D005240:Feasibility Studies; D005260:Female; D006801:Humans; D016014:Linear Models; D016015:Logistic Models; D008327:Mammography / Q000592:standards; D010173:Palpation / Q000592:standards; D011237:Predictive Value of Tests; D011300:Preoperative Care / Q000379:methods; D015203:Reproducibility of Results; D012189:Retrospective Studies; D018709:Statistics, Nonparametric; D016217:Ultrasonography, Mammary / Q000592:standards