Budgetary constraints can limit the number and types of modifications made to an existing program -- a given degree of speedup is worth only so much time and trouble.

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'''S''''''oftwareMaintenance'''
* Maintenance can be defined as four activities:
** Corrective Maintenance
*** A process that includes diagnosis and correction of errors. 
** Adaptive Maintenance
*** Activity that modifies software to properly interface with a changing environment (hardware and software). 
** Perfective Maintenance
*** Activity for adding new capabilities, modifying existing functions and making general enhancements.
*** This accounts for the majority of all effort expended on maintenance. 
** Proactive Maintenance (Relatively Rare)
*** Activity which builds software keeping in mind future maintainability or reliability
*** Ensuring that a basis exists which allows for future enhancements.
* http://hebb.cis.uoguelph.ca/~dave/27320/quality/maintain.html