From blank values to 0

Hello,

I want to calculate a standard deviation on these data :

KF1

Week1 Week2 Week3

A 5 8

B 6 9

C 1 5 6

My problem is with blank values because for the characteristics A and B, my deviation is just calculated for 2 weeks instead of 3 weeks like I would like.

It's necessary to replace blank values by 0 but it doesn't work with this formula : KF1 = KF1 - 0 or KF1 = KF1 + 1 - 1

Do you know a parameter in the Bex in V3.5 to do that or an other solution ?

Thanks in advance.

Christophe