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We empirically examine the relationship between software design modularity and software quality in open source software (OSS) development projects. Conventional wisdom suggests that degree of software modularity affects software quality. An analysis of 203 software releases in 46 OSS projects hosted on SourceForge.net lends support for a more complex relationship between software modularity and software quality than conventional wisdom suggests. We find that software modularity is associated with reduced software complexity, an increased number of static software bugs, and a mixed relationship with the percentage of bugs closed. We do not find empirical evidence supporting any relationship between modularity and other measures of customer satisfaction. In addition to empirically testing the relationship between modularity and quality, we introduce new measures of software modularity and software quality. Implications are developed for the theory of modularity and the practice of software development.