The rough membership function (RMS) expresses how strongly an element x belongs to the rough set X in view of information about the element expressed by the set of attributes B.
Consider the information system in Table 
.
| a | b | d | # objects | |
| 0 | 0 | 0 | 99 | |
| 0 | 0 | 1 | 1 |
The class is split into two decisions, but one decision value is dominating. Calculation of the RMS for the object in class gives the value 0.01. For one of the objects with the dominating decision value, it is 0.99. When the set X is a decision class and the objects in U are indiscernible, the RMS can be used as a measure of accuracy, or validity.