Rule based data object matching to determine if two or more objects are similar to allow the objects to be merged into one master data object. The rules explicitly state what conditions are required to consider two or more objects equal. The certainty of the rule may be specified. Examples for the certainty are automatic, manual high, medium and low probability. For example, the certainty "automatic" specifies that the objects can be considered matches and no additional manual verification is needed. The certainty "manual low probability" specifies that there is a low probability that the objects are matches and that a manual verification is needed. Matching rules may be grouped by priority. If matches are not found, the next highest priority group of rules is processed. Hierarchical matching rules may be specified that are able to express similarity of objects considering structured objects containing sub-objects in a hierarchical manner.