THE MEAN COST RATING (MCR) IS SOMERS-D - A METHODOLOGICAL NOTE

The purpose of this note is to show that the Mean Cost Rating (MCR), a particularly useful statistic for measuring the accuracy of instrumen ts used to predict recidivism, actually is a special case of Somers' D . This latter statistic, normally used to measure concordance in a con tingency table, is reported on the output of commonly used statistical packages. Consequently, researchers who wish to use the MCR can obtai n this statistic without any special programming or toilsome hand calc ulation. However, some caution must be exercised in obtaining the MCR from a statistical package because Somers' D has two forms, one with t he rows as the independent variable and the other with the columns as the independent variable. The MCR is the form of Somers' D that is opp osite to what might be expected.