I am trying to create a calculated measure that subtracts measures that are in different groups, but only if they have the same dimension member whatever that may be. (edit - essentially exclude UnknownMember numbers in the subtraction)

I have tried using Scope:

```
CREATE MEMBER CURRENTCUBE.[Measures].[CalcField] as NULL;

SCOPE([Dimension1].[DimensionField1].MEMBERS);
    [Measures].[CalcField] = [Measures].[a] - [Measures].[b];
END SCOPE;

```

I have also tried using a Tuple without success:

```
CREATE MEMBER CURRENTCUBE.[Measures].[CalcField]
AS ([Measures].[a] - [Measures].[b], [Dimension1].[DimensionField1]);

```

But I think there is something fundamental that I am missing

**Edit**

col a b dimension  

1. 9 0 x  

2. 0 2 x  

3. 1 5 null 

If you aggregate those rows I want the answer 7 not 5 

Using a simple CASE WHEN works if the dimensions in use, but otherwise it blindly subtracts everything again

```
CASE 
  WHEN [Dimension1].[DimensionField1] IS [Dimension1].[DimensionField1].UnknownMember THEN 0 
  ELSE [Measures].[a] - [Measures].[b]
END

```

Using Aggregate works at the high level, but then when I use the Dimension I get no per member results

```
Aggregate(
  EXCEPT(
    [Dimension1].[DimensionField1].Members, {[Dimension1].[DimensionField1].UnknownMember, [Dimension1].[DimensionField1].[All]}
  ),[Measures].[a]) 
- 
Aggregate(
  EXCEPT(
    [Dimension1].[DimensionField1].Members, {[Dimension1].[DimensionField1].UnknownMember, [Dimension1].[DimensionField1].[All]}
  ),[Measures].[b]) 

```

**Workaround Solution**  

I am sorry if I was not clear on my problem, but I have ended up solving this problem by putting the data I need in at the ETL stage rather than calculating it in the Cube