The intuition behind the above expression is as follows: The term formula_31 represents the cost (in bits) of optimally encoding (or transmitting) feature information when it is known that the objects to be described belong to category formula_22. Similarly, the term formula_33 represents the cost (in bits) of optimally encoding (or transmitting) feature information when it is known that the objects to be described belong to category formula_28. The sum of these two terms in the brackets is therefore the weighted average of these two costs. The final term, formula_35, represents the cost (in bits) of optimally encoding (or transmitting) feature information when no category information is available. The value of the category utility will, in the above formulation, be negative (???).