The present invention discloses a system and a method for estimating a measure of confidence in a match generated from a case-based reasoning system. The measure of confidence is determined by using a confidence function that is derived for all existing cases in the case base. The confidence function is then used to estimate the confidence of any matches between a new case and an existing case. In the present invention, a retrieval unit uses a similarity index and retrieves any existing cases from the case base that has a measurement of similarity to the new case that is within a predetermined similarity threshold. The confidence function then maps the similarity measurement for each retrieved case to a corresponding measure of how many different outcomes are likely given the level of similarity. A report of the existing cases that have the best measures of confidence are then provided in a list. In another embodiment, the confidence estimation is used to analyze an error log generated from a malfunctioning machine.