sum all rating field value

hello all,

here i create a pic list having four value like (1, 2, 3, 4) and name of pic list is rating .

after that i assign rating for each employee like 

emp1         1

emp2         2

emp3         3

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now i want to calculate overall rating like ( sum of all rating / count of all rating ) , how can i do this even i was used 

formula like   " return #CALC_SUM.Appraisal( Rating ) / #CALC_COUNT.Appraisal( Rating ) ; " but still facing a problem ....any help  ...?

thanks.