It is common to retain expert witnesses in civil and business litigation. Expert witnesses are people who possess a special knowledge in a particular area that is not know, or beyond the common experience of a lay person.
I have a case where there is an issue of emotional distress damages due to post traumatic stress. To prove damages in cases like that it will be necessary to hire a psychologist or psychiatrist to opine. What you like to obtain is some objective test to determine the existence of such an injury. There a quantitative as well as qualitative opinions. I always feel it is necessary to strive for some sort of quantitative measure that cannot be easily disputed.