When a new computer software package is developed, a testing procedure is often put into effect to eliminate the faults, or bugs, in the package. One common procedure is to try the package on a set of well-known problems to try to see if any errors result. This goes for some fixed time with all detected errors being noted. Then the testing stops and the package is carefully checked to determine the specific bugs that were responsible for the observed errors, and the package is then altered to remove these bugs. A problem of great importance is the estimation of the error rate of this revised software package.