Conservative estimates indicate that, in large systems, approximately half of all software errors are found in 15% of the modules, and 80% of all software errors are found in 50% of the modules. More dramatic results from Gary Okimoto and Gerald Weinberg indicate that 80% of all errors were found in just 2% of the modules. Thus, when testing software, you might consider that, where you find errors, you will probably find more.
Maintain logs not only of how many errors are found per time period for the project, but also how many errors are found per module. When history shows a module to be highly error-prone, you are probably better off rewriting it from scratch, with an emphasis on simplicity rather than cleverness.
References:
Okimoto, G. and Weinberg, G., Quality Software Management, Vol. 1: Systems Thinking, New York: Dorset House, 1992.
Endres, A., "An Analysis of Errors and Their Causes in System Programs," IEEE Transactions on Software Engineering, June 1975.
