  Various issues related to the complexity of apprais- ing the capabilities of
physics models implemented in Monte Carlo simulation codes and the evolution of
the functional quality the associated software are considered, such as the
dependence on the experimental environment where the software operates and its
sensitivity to detector characteristics. The concept of software validity as
relative to the environment is illustrated by means of a real-life experimental
test case. Methods and techniques to mitigate the risk of deteriorating the
quality of the software are critically discussed: they concern various
disciplines of the software development process. Quantitative validation of
physics models is advocated as a method to appraise their capabilities
objectively and to monitor the evolution of their associated software behavior.
