title: Measures of risk and their relationship to the relative size of a high-risk group: application to medical thromboprophylaxis.
abstract: BACKGROUND
  The aim of this study was to establish the meaning of "high-risk" when the subgroup so defined by risk factor analysis is a substantial proportion of the population. This is clinically important when patients, deemed to be at high risk as a result of risk factor analysis, become eligible for a clinical intervention to decrease the risk, especially if the intervention has adverse effects. One example in clinical practice is the assessment of eligibility for medical thromboprophylaxis.
  
  
  METHODS
  Equations were derived relating risk and the proportion of the population (F) deemed to be at high risk on risk factor analysis, based on the formula for weighted average. The equations were validated for the population of medical inpatients at high- or low-risk of thromobembolic events using a spreadsheet model of thrombosis risk containing known risk factors for venous thromboembolism in this population.
  
  
  RESULTS
  The validated equations define an upper limit of absolute and incremental risk (risk relative to the whole population) in the high-risk group that is a function of or equal to 1/F, respectively. The added risk in the high-risk group declines to zero as F tends to 1, because it must be balanced by the diminishing risk in the progressively smaller low-risk group while maintaining the population average.
  
  
  CONCLUSIONS
  The results of this study have implications for the validity of the published eligibility criteria for medical thromboprophylaxis.
authors.affiliation: Medical Department, Albany Regional Hospital, Albany, Western Australia, Australia.
authors.forename: J Alasdair
authors.initials: JA
authors.lastname: Millar