[1] It seeks to determine whether the new assessment adds information that cannot be obtained with simpler, already existing methods.
[3] If the instrument is less successful than an existing tool, then it is incrementally invalid.
Incremental validity is usually assessed using multiple regression methods, involving a regression model with other variables fitted to the data and another model with the focal variable added.
Using the change in R-square is more appropriate than mere raw correlations, because the raw correlations do not reflect the overlap of the newly introduced measure and the existing measures.
[3] For example, the College Board has used multiple regression models to assess the incremental validity of a revised SAT test.