International Encyclopedia of Systems and Cybernetics

In rules based adaptive systems, as for instance pattern recognition machines, but also, in a way more difficult to put in evidence, in biological and probably social systems, there is a constant need to evaluate the relative success of the rules. J. HOLLAND calls this process "credit assignment" and discusses it as follows: "Credit assignment is not particularly difficult where the situation provides immediate reward or precise information about correct actions. Then the rules directly involved are simply strengthened. Credit assignment becomes difficult when credit must be assigned to early acting rules that "set the stage", making possible later useful actions. Stage setting moves are the key to success in complex situations, such as in playing chess or investing resources. The problem is to credit an early action, which may look poor (such as the sacrifice of a piece in chess) but which set the stage for later positive actions (such as the capture of a major piece in chess). When many rules are active simultaneously, the problem is exacerbated. It may be that only a few of the early acting rules contribute to a favorable outcome, while others, active at the same time, are ineffective, or even obstructive. Somehow, the credit assignment algorithm must sort this out, modifying rule strengths appropriately.

"Credit assignment in classifier systems is based on competition…. That is, when a rule wins the competition, it actually "pays" its bid to the rule(s) that sent the message(s) satisfying its conditions. The rule acts as a kind of go-between or broker in a chain that leads from the stage-setting situation to the favorable outcome" (1992, p.176-7).

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