The external quality attributes of software product are influenced by the internal and structural properties of the design of software products. This implies that to estimate the external quality, the internal properties need to be accurately measured. Measuring the internal attributes requires the use of metrics that are both theoretically and empirically valid. This work identifies and theoretically validates an OO metrics that measures polymorphism. The property-based and measurement theory-based validation approaches are used to show that the metric measures the concept of complexity which has impact on the external quality attribute of software design. © 2014 IEEE.