crements. The relative clarity of requirements may influence an increment plan in two ways. Volatile requirements may be implemented in an early increment, so they can be clarified. Alternatively, unstable requirements may be planned for later implementation, when questions affecting the requirements have been settled.

Increasingly, customers in the commercial sector are specifying formal software reliability requirements. There are methods that can be used to compute the reliability needed for each subsystem to achieve a system-wide reliability. Subsystems with the highest reliability requirements may be candidates for early increments. A functional usage distribution is developed as part of a top-level cleanroom specification. Expected usage probabilities of system functions are established from historical data and estimates provided by customers. System functions with high expected usage probabilities will receive greatest exposure in the field, and they should therefore benefit from the greatest exposure to testing. Since increments are cumulative, the functions developed in early increments will be tested every time a new increment enters the testing process. System functions expected to receive the greatest operational usage by customers are therefore candidates for early increments. Some functions expected to receive low usage may even be regarded as optional and scheduled for development in the final increment if time permits.

Systems that involve both hardware and software must be developed as a coordinated effort between hardware and software engineers, and incremental development is an ideal framework for this coordination.

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