1. A processor-based method for determining difficulty measures for training cases used in developing a solution to a problem, comprising: providing a set of training cases having respectively associated difficulty measures; operating, by a processor, a candidate solution on a particular training case; determining, by the processor, a performance measure of the candidate solution operating on the particular training case; determining, by the processor, a credibility rating of the candidate solution with respect to the particular training case, the credibility rating indicating a degree to which the performance measure is representative of the difficulty measure of the particular training case, wherein determining the credibility rating of the candidate solution with respect to the particular training case is based on inputs that exclude the performance measure of the candidate solution operating on the particular training case; and modifying, by the processor, the difficulty measure of the particular training case based on the performance measure of the candidate solution operating on the particular training case and the credibility rating of the candidate solution.