The foremost expert on psychopathy today is a psychologist named Robert Hare (1934–). He has developed an intensive interview to measure psychopathy named the Hare Psychopathy Checklist. His first version was published in 1980 and the revised version (PCL-R) in 1991. The PCL-R is a twenty-item clinical rating scale that is scored based on information from a semi-structured interview and available legal files and medical records. Collateral interviews with someone who knows the subject well are also conducted, as psychopathic individuals are not always reliable informants. Although the maximum possible score is 40, the average scores in both male and female offender populations range from about 22 to 24. Hare uses a cut-off score of 30 to distinguish psychopaths from non-psychopaths. He believes that psychopathy is more of a category than a dimension. This means that someone either is or is not a psychopath. Notably, some other researchers disagree and believe that psychopathic traits fall on a continuum.