Introduction: Markers that purport to distinguish subjects with a condition from those without a condition must be evaluated rigorously for their classification accuracy. A single approach to statistically evaluating and comparing markers is not yet established.
Methods: We suggest a standardization that uses the marker distribution in unaffected subjects as a reference. For an affected subject with marker value Y, the standardized placement value is the proportion of unaffected subjects with marker values that exceed Y.
Results: We apply the standardization to two illustrative datasets. In patients with pancreatic cancer placement values calculated for the CA 19-9 marker are smaller than for the CA-125 marker, indicating that CA19-9 is a better marker. For detecting hearing impairment, the placement values for the test output (the marker) are smaller when the input sound stimulus is of lower intensity. This indicates that the test better distinguishes hearing impaired from unimpaired ears when a lower intensity sound stimulus is used.
Explicit connections are drawn between the distribution of standardized marker values and the receiver operating characteristic (ROC) curve, one established statistical technique for evaluating classifiers.
Discussion: The standardization is an intuitive procedure for evaluating markers. It facilitates direct and meaningful comparisons between markers. It also provides a new view of ROC analysis that may render it more accessible to those as yet unfamiliar with it. The general approach provides a mechanism to statistically address important questions that are typically not addressed in current marker research, such as quantifying and controlling for covariate effects
