The Resnik (res) measure simply uses the Information Content of the LCS as the similarity value: The Resnik measure is considered somewhat coarse, since many different pairs of concepts may share the same LCS.
it is less likely to suffer from zero counts (and resulting undefined values) since in general the LCS of two concepts will not be a very specific concept (i.e., a leaf node in WordNet), but will instead be a somewhat more general concept that is more likely to have observed counts associated with it.