The AMA Fifth (5th) Edition Permanent Impairment guidelines mandates that an person's impairment is calculated under a formula that combines impairments from multiple body parts/functions and how it impairs the person as a whole.  In essence, it is a ratio of how an impairment affects the entire body.  You can see my video on "Personal Injury Case Value" where I briefly explain this ratio process and give specific examples.  Click on my video center to locate video.