Doddington's Zoo: Sheep, Goat, Lamb, Wolf

Sheep – Sheep comprise our default speaker type. In our model, sheep dominate the population and systems perform nominally well for them. 

Goats – Goats, in our model, are those speakers who are particularly difficult to recognize. Goats tend to adversely affect the performance of systems by accounting for a disproportionate share of the missed   detections. The goat population can be an especially important problem for entry control systems, where it is important that all users be reliably accepted.

Lambs – Lambs, in our model, are those speakers who are particularly easy to imitate. That is, a randomly chosen speaker is exceptionally likely to be accepted as a lamb. Lambs tend to adversely affect the performance of systems by accounting for a disproportionate share of the false alarms. This represents a

potential system weakness, if lambs can be identified, either through trial and error or through correlation with other directly observable characteristics.

Wolves – Wolves, in our model, are those speakers who are particularly successful at imitating other speakers. That is, their speech is exceptionally likely to be accepted as that of another speaker. Wolves tend to adversely affect the performance of systems by accounting for a disproportionate share of the false alarms. This represents a potential system weakness, if wolves can be identified and recruited to defeat systems.

Ref:

Doddington, George, et al. Sheep, goats, lambs and wolves: A statistical analysis of speaker performance in the NIST 1998 speaker recognition evaluation. NATIONAL INST OF STANDARDS AND TECHNOLOGY GAITHERSBURG MD, 1998.