Reduction of knowledge is one of the important issues in the research of rough set theory. It has been proven that the search for an optimal (minimal) reduction of decision table is NP-hard problem. Thus people have been trying to search for more efficient heuristic algorithms to get an approximate reduction of decision table all the time. Every heuristic must depends on a measure of the significance of attributes in decision table, because it is the key heuristic information to determine which attribute is reducible in decision table. It was analyzed by examples in this paper that the current four commonly used methods all have some drawbacks. Using them to a heuristic will lead to wrong results. Then a new definition for measuring the significance of attributes in decision table is given based on discernibility matrix. The new definition can discern the difference of significance of no-core attributes what cannot be discerned or misled by current commonly used methods. The new definition is very useful in the practice of data mining and reduction of knowledge.