It has been discussed in chapter 3 that the ISO/IEC 9126 standard [ISO99] defines software quality as composed of six external characteristics – functionality, reliability, usability, efficiency, maintainability and portability. For an Entity-Relationship Diagram, maintainability can be assessed by its structural complexity and the structural complexity itself can be determined by the two sub-characteristics called understandability and modifiability. As discussed in section 3.5, Genero et al. [Genero et al. 2002] identified some measures which have a high correlation with the understandability time and the modifiability time. However, their research was restricted to specifying the correlation and it did not specify any metric. In chapter 3, we have concluded that there is a need to define these characteristics quantitatively as a metric which can be easily applied to practical problems.