creating a KPI % calcualtion using null values vs values

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I have a report that shows members details, I want to add a KPI measure (%) showing members with null values in various feilds. Say if there is 10 members and 5 have no address details so 50% are complete. Can I do this as the report formats? If so whats the best way? How would I "count" the null values and values to create the calculation?

Cheers

 

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error occured

Pat, got the error 'the expression you entered has a function containing the wrong number of arguments"

I tried to use: =Sum(IsNull([address2]), 1, 0)/Count(*)
in the group footer

I have attached a sample of the report. Cheers