On the basis of the improved materials, many factors affecting the concept difficulty were generalized as the following two fundamental factors: attribute saliency (AS) and attribute frequency (AF). The more salient the relevant attribute, or the more frequent it appears in the positive examples, the easier the concept, and vice versa. When the AS or AF of an irrelevant attribute is greater, it will disturb the correct concept formation. In addition, a somewhat strange result was noticed that at least when there are only 2-3 relevant attributes, the number of relevant attributes does not affect the concept difficulty
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