An apparatus and method for preventing a computation error of band-by-band bit allocation between an encoder and a decoder. The applied drain voltage while sensing the data of one of the storage parts depends on the data stored at the other storage part; the different parts can be in different, neighboring memory cells. In response to the at least one result, at least one act is performed selected from the group consisting of: denying the request to change from the first context to the second context; requesting the user to provide information relating to the requested change; and requesting the user to affirm information relating to the requested change.