Domain Range Ratio (DRR)

The Domain Range Ratio (DRR) of a logical function (or software component) is the ratio of the cardinality of its set of all possible inputs (domain) to the cardinality of its set of all possible outputs (range). Most developers try to reduce the DRR of their functions to as small as possible.

Example

To understand this metric, consider the function isEven():

boolean isEven(short x) {
    if (x == 0) {
        throw new Exception("Give me a proper number");
    }
    return x % 2 == 0;
}

This function checks if a short number is even or not, with an exception at 0. The DRR of isEven() is 64k possible inputs to 2 outputs (boolean, exception). A high DRR, like this one, indicates potential information loss and a low probability of finding defects just by selecting a handful of samples.