Animated agents are becoming increasingly frequent in research and applications in speech science. An important challenge is to evaluate the effectiveness of the agent in terms of the intelligibility of its visible speech. In three experiments, we extend and test the Sumby and Pollack (1954) metric to allow the comparison of an agent relative to a standard or reference, and also propose a new metric based on the fuzzy logical model of perception (FLMP) to describe the benefit provided by a synthetic animated face relative to the benefit provided by a natural face. A valid metric would allow direct comparisons accross different experiments and would give measures of the benfit of a synthetic animated face relative to a natural face (or indeed any two conditions) and how this benefit varies as a function of the type of synthetic face, the test items (e.g., syllables versus sentences), different individuals, and applications.