An esper rating or esper potential appears to be a measure of a person's psionic abilities. Commander Hayes mentions esper ratings at least twice. One time is when the Pkunk are absorbed by the Yehat. He says that certain crew members with high esper ratings fell unconscious, overcome by a feeling that something very wrong had happened. Another time he mentions it is when The Captain brings the Taalo Shield in for study. Hayes claims that whenever you put a charge anywhere through the shield, all crewmen with high esper potentials get a bad headache, and "all evidence of psychon interaction is flatlined." Besides the six people on the Earth Starbase who react to the Pkunk disappearance, Ensigns Hodgkins and Witherspoon, who reacted to the Taalo Shield, are the only crew members specifically identified as having noticeable esper ratings.