Page 1404 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

model the software development process while taking into account the
necessity of returning to previous phases to correct system faults.

However, one of the major criticisms of this model is that it allows the
developers to step back only one phase in the process. It does not
make provisions for the discovery of errors at a later phase in the
development cycle.

The waterfall model was improved by adding validation

and verification steps to each phase. Verification evaluates the
product against specifications, whereas validation evaluates how
well the product satisfies real-world requirements. The improved

model was labeled the modified waterfall model. However, it did
not gain widespread use before the spiral model dominated the
project management scene.

Spiral Model

In 1988, Barry Boehm of TRW proposed an alternative lifecycle model
that allows for multiple iterations of a waterfall-style process. Figure
20.3 illustrates this model. Because the spiral model encapsulates a
number of iterations of another model (the waterfall model), it is
known as a metamodel, or a “model of models.”