The implementer of a large, complex software system cannot make it compIetely defect free,
so he must normally provide fixes for defects found after the code is put into service. A
system user may do preventive service by installing these fixes before the defects cause him
problems. Preventive service benefits a user to the extent that it avoids costly operational
problems, but it costs him the resource required to prepare, disseminate, and install fixes; and
fixes tbemseIves can introduce errors.
The benefit from removing a given defect depends on how many problems it would otherwise
cause. Benefits may be estimated by modelling problem occurrence as a random process in
execution time governed by a distribution of characteristic rates, It is found that most of the
benefit to be realized by preventive service comes from removing a relatively small number of
high rate defects that are found early in the service life of the code, and that corrective service
is preferable to preventive service as a way of dealing with most defects found after code has
had some hundreds of usage months of service.
By: E. N. Adams
Published in: RC8228 in 1980
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