  Open Source Software (OSS) Projects are gaining popularity these days, and
they become alternatives in building software system. Despite many failures in
these projects, there are some success stories with one of the identified
success factors is modularity. This paper presents the first quantitative
software metrics to measure modularity level of Java-based OSS Projects called
Modularity Index. This software metrics is formulated by analyzing modularity
traits such as size, complexity, cohesion, and coupling of 59 Java-based OSS
Projects from sourceforge.net using SONAR tool. These OSS Projects are selected
since they have been downloaded more than 100K times and believed to have the
required modularity trait to be successful. The software metrics related to
modularity in class, package and system level of these projects are extracted
and analyzed. The similarities found are then analyzed to determine the class
quality, package quality, and then combined with system architecture measure to
formulate the Modularity Index. The case study of measuring Modularity Index
during the evolution of JFreeChart project has shown that this software metrics
is able to identify strengths and potential problems of the project.
