Overall competencies rating calculation

Content

Hello,

I configured an average calculation related to the overall competencies rating. In this way, the system calculate in the average also competencies without a rating. For example I have 3 competencies:

comp a: rating 3

comp b: rating 4

comp c: no rating

The system calculates the overall competencies rating as (3+4+0)/3. Is there a way to exclude from the calculation method the competencies without a rating? So with this formula (3+4)/2

Thanks

Valeria