ABSTRACT
It took a while before software engineers became aware
of the inherent limitations of this software process model,
namely, the fact that the separation in phases was too strict
and inflexible, and that it is often unrealistic to assume that
the requirements are known before starting the software
design phase. In the late 1970s, a first attempt was made
toward a more evolutionary process model, the so-called
“change mini-cycle” as proposed by Yau et al.[4]
Also in the 1970s, Manny Lehman started to formulate
his laws of software evolution (see Table 1). The postulated
laws were based on earlier work carried out by Lehman to
understand the change process being applied to IBM’s OS
360 operating system. His original findings were confirmed
in later studies involving other software systems.[5] This was
probably the first time that the term software evolution was
explicitly used to stress the difference with the postdeploy-
ment activity of software maintenance.
