"Calculating average with missing values"

Hi everyone,

Having a Dataset like the following one:

row - Att1 - Att2 - Att3

(1)     5        7         5     

(2)     6        8         9     

(3)               7         6

(4)               7         

I want to calculate the average so that result would be the following one:

row - Att1 - Att2 - Att3 - Average

(1)     5        7         5         5.6

(2)     6        8         9         7.6

(3)               7         6         6.5

(4)               7                    7

I am using operator Generate Attributes, with the following function:

avg(Att1, Att2, Att3)

However, when any of the rows has a missing value, I simply get a Missing Value as Average.

I would like to know how to solve this problem.

Thanks in advance