title: Comparing methods of measurement: why plotting difference against standard method is misleading.
abstract: When comparing a new method of measurement with a standard method, one of the things we want to know is whether the difference between the measurements by the two methods is related to the magnitude of the measurement. A plot of the difference against the standard measurement is sometimes suggested, but this will always appear to show a relation between difference and magnitude when there is none. A plot of the difference against the average of the standard and new measurements is unlikely to mislead in this way. We show this theoretically and by a practical example.
authors.affiliation: Department of Public Health Sciences, St George's Hospital Medical School, London, UK.
authors.forename: J M
authors.initials: JM
authors.lastname: Bland
authors.forename: D G
authors.initials: DG
authors.lastname: Altman
mesh_terms: D015982:Bias; D006801:Humans; D012015:Reference Standards; D012016:Reference Values; D013223:Statistics as Topic; D014894:Weights and Measures