Prior studies show it to be a reasonably effective metric.
In learning mode, cases were also retrieved on the same basis. However, in this mode, the failure reasons attached to the case were used to censor its retrieval. Each time that a case was retrieved in learning mode, these failure conditions were also tested. If each failure reason was not satisfied in the new problem specification, the retrieval mechanism returned the case for replay. If, on the other hand, a failure reason was found to be true in the new problem context, then the case that repaired the failure was retrieved.