The Duke-Heidelburg quotient was, in the field of 23rd century psychology, a relative means determining the potential in ones abilities of possessing extrasensory perception; especially when used in correlation with the esper rating, aperception quotient, and general knowledge quotient.
According to a readout on Dr. Elizabeth Dehner in 2265, she had a Duke-Heidelburg quotient rating of 256. Her ratings were considered "better than average."
Likewise, Gary Mitchell had a Duke-Heidelburg quotient rating of 261. His ratings reached into the next level of "well above average." (TOS: "Where No Man Has Gone Before")