Patients don't have an "individual risk" or unique probability of an outcome. Outside Mendelian inheritance, risks are conditional probabilities and differ as the risk factors included differ, at times substantially. This lack of reliability is an inherent limitation and is not resolved by including additional risk factors. Groups of like individuals need to be assembled to measure the probability of an outcome. Many groups, like any individual, can be identified, eg, groups of the same age, sex, race, or any combination of these attributes (or any others). That each of these groups may have different risk means there is no such thing as individual risk. This issue was identified by John Venn in 1866 and is known as the reference class problem. Models relate risk factors to outcomes in populations. The number calculated for an individual should not be reported as their individual or true risk, nor should it be used as the sole criterion for clinical decisions. Instead, Feinstein proposed relying on clinically important subgroups. An example would be utilizing an individual's blood pressure as the primary determinant of hypertension treatment decisions, not an unreliable individual risk estimate.
© 2012 Wiley Periodicals, Inc.