Weighted averages that omit columns with N/A

Thanks, PReagan but that didn't work for me. I want the Overall Rating to reflect an average that doesn't penalize individuals that do not have a score in a certain Performance Review category (i.e. 'Key Responsibility' may not apply to a position, so they only score themselves under 'Competencies') similar to a student missing an assignment and having a doctor's excuse or other pass to excuse the assignment.