1. A method, in a data processing system comprising a processor and a memory, for answering questions about at least one previous execution of a question answering (QA) system on a previous input question, comprising: receiving, in the data processing system, an input question directed to a previous execution of a QA system with regard to a previous input question; processing, by the data processing system, the input question to generate at least one query for application to a corpus of information, wherein the corpus of information comprises information about the QA system and the previous execution of the QA system on the previous input question; applying, by the data processing system, the at least one query to the corpus of information to generate candidate answers to the input question; and outputting, by the data processing system, a final answer for the input question based on the candidate answers; wherein the input question is directed to identifying differences between the previous execution of the QA system and another previous execution of the QA system, and wherein the candidate answers identify possible sources of differences between the previous execution of the QA system and the another previous execution of the QA system.
6. The method of claim 1 , wherein applying the at least one query to the corpus of information to generate candidate answers to the input question comprises consulting a knowledge base data structure specifying information indicating probable causes of changes in results of runs of the QA system.