1.1.2. Incremental development

Several recent books, e.g. [Brooks75-95], [Meyer95], [Glass95], argue in favour of a model of software process based on incremental development. This is indeed neither new, nor a priori bound to object-orientation.

The process consists in producing a simple running system at the most early stage of the development, then refining it progressively and taking care that it remains executable after every step. The distinctive character of this process model is that it does not define phases of different kinds.

The motivations for incremental development include:

• Better track changes in the environment, and in the requirements; allow even for progressive refinements in the requirements, i.e. accept that the requirements are initially not fully specified or otherwise incomplete. The rationale for this liberality is the pragmatic view that requirements will anyway be incomplete.
• Allow for early testing and user feed-back. This provides validation based on empiric data, as opposed to verification of conformance to specifications. This last option does not validate the specifications themselves, with the results that the validity of these specifications becomes more critical, and that more effort has to be spent into producing them. Such a policy becomes suboptimal when significant amounts of expertise are effectively acquired during the development, with respect to what was available at the time when the specifications were written. This situation is in turn archetypical of programming-in-the-large.
• Track the risk of over-specification. This risk rises if the process requires that the specifications are defined fully at an early stage of the development.

A necessary condition, for incremental development to be reasonable, is that increments of comparable importance, occurring at various stages of the development, involve an equivalent amount of effort.

This rules out in the long range any validation strategy based on regression testing, since the mass of these tests continually increases.

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