AbstractÐA central feature of the evolution of large software systems is that changeÐwhich is necessary to add new functionality, accommodate new hardware, and repair faultsÐbecomes increasingly difficult over time. In this paper, we approach this phenomenon, which we term code decay, scientifically and statistically. We define code decay and propose a number of measurements (code decay indices) on software and on the organizations that produce it, that serve as symptoms, risk factors, and predictors of decay. Using an unusually rich data set (the fifteen-plus year change history of the millions of lines of software for a telephone switching system), we find mixed, but on the whole persuasive, statistical evidence of code decay, which is corroborated by developers of the code. Suggestive indications that perfective maintenance can retard code decay are also discussed.

Software is invisible, disappearing into les on disks. The invisible nature of software contributes to low programmer productivity b y hiding system complexity, particularly for large team-oriented projects. Visualization can help software engineers cope with this complexity and thereby increase programmer productivity. We describe four innovative visual representations of software that scale to productionsized systems and illustrate their usage in ve software case studies involving: version history, di erencing, static properties, performance pro les, and dynamic program slices.

Visualizations of software changes are presented that complement existing visualizations of software structure. The principal metaphors are matrix views, cityscapes, bar and pie charts, data sheets and networks. Linked by selection mechanisms, multiple views are combined to form perspectives that both enable discovery of high-level structure in software change data and allow effective access to details of those data. Use of the views and perspectives is illustrated in two important contexts: understanding software change by exploration of software change data and management of software development.

Program slicing is an automatic technique for determining which code in a program is relevunt to a particular computation. Slicing has been applied in many areas, including program understanding, debugging, and maintenance. However, little attention has been paid to suitable interfaces for exploring program slices. We present an intelface for program slicing that allows slicing at the statement, procedure, orBle level, ana' provides fast visual feedback on slice structure. Integral to the intelface is a global visualization of the program that shows the extent of a slice as it crosses procedure ana'jle boundaries, and facilitates quick browsing of numerous slices.

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