Article Text

Download PDFPDF
Constructing diagnostic likelihood: clinical decisions using subjective versus statistical probability
  1. John Kinnear1,2,
  2. Ruth Jackson1
  1. 1Postgraduate Medical Institute, Anglia Ruskin University, Chelmsford, UK
  2. 2Department of Anaesthetics, Southend University Hospital NHS Foundation Trust, Southend, UK
  1. Correspondence to Professor John Kinnear, Postgraduate Medical Institute, Anglia Ruskin University, Bishop Hall Lane, Chelmsford, CM1 1SQ, UK; John.Kinnear{at}


Background Although physicians are highly trained in the application of evidence-based medicine, and are assumed to make rational decisions, there is evidence that their decision making is prone to biases. One of the biases that has been shown to affect accuracy of judgements is that of representativeness and base-rate neglect, where the saliency of a person's features leads to overestimation of their likelihood of belonging to a group. This results in the substitution of ‘subjective’ probability for statistical probability.

Methods This study examines clinicians' propensity to make estimations of subjective probability when presented with clinical information that is considered typical of a medical condition. The strength of the representativeness bias is tested by presenting choices in textual and graphic form. Understanding of statistical probability is also tested by omitting all clinical information.

Results For the questions that included clinical information, 46.7% and 45.5% of clinicians made judgements of statistical probability, respectively. Where the question omitted clinical information, 79.9% of clinicians made a judgement consistent with statistical probability. There was a statistically significant difference in responses to the questions with and without representativeness information (χ2 (1, n=254)=54.45, p<0.0001).

Conclusions Physicians are strongly influenced by a representativeness bias, leading to base-rate neglect, even though they understand the application of statistical probability. One of the causes for this representativeness bias may be the way clinical medicine is taught where stereotypic presentations are emphasised in diagnostic decision making.


Statistics from

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.


  • Contributors JK conceived and designed the study, collected the data and drafted the paper. RJ analysed the data and reviewed the drafted paper.

  • Competing interests None declared.

  • Ethics approval Faculty of Medical Science Research Ethics Panel for Anglia Ruskin University (FREP no 15/16 085).

  • Provenance and peer review Not commissioned; externally peer reviewed.