I'm not a statistician, but I believe that they extrapolate their findings in smaller populations who are tested to the population at large. Although narcissists are prevalent in the media and discussed on social media, they still are a small minority. Also, not all abusers have NPD. They may be sociopaths, addicts, or have BPD or another diagnosis, or be "sub-clinical" narcissists. Learn more in my book and about the interrelationship with codependents. Dating, Loving, and Leaving a Narcissist: Essential Tools for Improving or Leaving Narcissistic and Abusive Relationships https://whatiscodependency.com/dating-loving-and-leaving-a-narcissist/