A system is said to be extensible; if any modifications can be made to any of the existing system functionalities in addition of new functionalities with minimum impact. To attain extensibility, it has to be scheduled properly starting from the initial stage of the system development life cycle. Keeping in mind all the probable upcoming modifications to be made, the software designer should select the appropriate design patterns and finish the design for the application. As soon as the application design is finished, it must be examined to make sure that the application is extensible. This research paper recommends a research framework for Extensibility evaluation process and does an extensive review on extensibility of object oriented software. A metrics based Extensibility Evaluation Model for Object Oriented Design" has been recommended by creating the relationship in the middle of design properties. Consecutively researchers justifying the model correlation with the help of statistical measures, which shows that for all the System, Coupling, Cohesion, Inheritance, Polymorphism are highly correlated with Extensibility. The developed model has been authenticated by experimental tryout. Finally, it incorporates the empirical validation of the extensibility evaluation model as the author"s most important contribution. In Conclusion, Hypothesis testing is performed to test the significance of r (Correlation Coefficient) using 2-tailed test for a 95% confidence interval with different degrees of freedom. As a result, the researcher"s claim of correlating Extensibility with Coupling, Cohesion, Inheritance and Polymorphism at design phase is statistically extremely justified.

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Software program developers need to go from beginning to ending and understand source code of the program and other software attributes. The software complexities and length of the program exceedingly affects many design level quality attributes, specifically Simplicity, Testability and software Maintainability. Incomplete design of any software generally leads to misunderstanding and ambiguities and therefore to gives faulty design and development results. This is mainly seeming and appears owing to the absence of it's an appropriate observation, design and development control. However, high level design and program simplicity are very necessary and one of the vital attributes of the system development cycle. This research paper highlights the impact and significance of design level software simplicity in common and as a one of the most useful key factor or index of software quality assurance and testing. In this research work principally there are three major efforts are made. As a first contribution, a valuable relationship between software design quality factor simplicity and related object oriented design properties, this has been set up. In the second contribution, using design level corresponding metrics a simplicity evaluation model for object oriented software is developed. Subsequently, the developed simplicity model has been rationally authenticated by means of experimental data try-out.

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