The information measure is an objective measure of the quality of a classification and results from an information transmission view of the classification problem. So far information measures have only been derived for the case where an explicit assumption is made about the form of the distribution of attribute values within a class. One important method which involves no such explicit assumption is single link. In this paper we derive a new information measure which is optimised by classifications produced by the single link method. By investigating the properties of this information measure we are able to gain more insight into the single link method and also determine the type of problem to which it best applies.