Selectivity and specificity are performance characteristics of analytical methods which are frequently used in analytical literature. In general, the terms are applied verbally and a quantification of selectivity and specificity is given rarely. Excepted are methods like chromatography and ISE sensoring which use individual quantities such as selectivity coefficients, indices and other parameters to characterize analytical procedures and systems. Here a proposal is given to characterize selectivity and specificity quantitatively by relative values in a range of 0 to 1 expressing so a certain degree of selectivity and specificity. By examples it will be shown that the derived quantities characterize analytical methods and problems in a plausible way.