SSAS Tabular Calculation Groups – avoid using SELECTEDMEASURE or ISSELECTEDMEASURE
There is a very serious limitation in the behaviour of calculation groups when using the SELECTEDMEASURE or ISSELECTEDMEASURE functions, and we recommend not using them. Why? If a user creates their own custom calculations within their Power BI report (or composite model) then the value of SELECTEDMEASURE changes, breaking your calculation group logic.
Let me explain with an example:
In a tabular cube we may look to use calculation groups to apply additional filters when calculating measures. In our example we will be using a retail scenario, with budgets as well as ‘Like for Like’ (LFL) budgets. For those not in retail, LFL represents stores that were also open this time last year. We start with the measure “Budget” with the following definition:
We also have a hidden measure called “Budget LFL” with the following definition:
The measure “Budget LFL” is a variation of the “Budget” measure with additional filters applied at source. We also have a calculation group called “LFL Filter”, this is used to return the hidden “Budget LFL” measure based on the option selected in this filter and has the following definition:
This functionality is demonstrated below where the two visuals from Power BI show the same measure with and without the LFL Filter applied:
A problem arises when you try to create a custom measure (in the cube or within a Power BI report) and filter by the calculation group, for example we create a measure called “CustomBudget” which is a copy of the “Budget” measure and has the definition:
Adding this custom measure to the visual shown earlier we can see that the calculation group “LFL Filter” has no affect on the custom measure:
This is because the SELECTEDMEASURE() is now [CustomBudget] and not [Budget], therefore the logic in the calculation group doesn’t recognise the selected measure, and therefore doesn’t switch to the LFL measure.
To get around this we move the bulk of the logic from the calculation group to the measure itself. The measure now changes its behaviour by looking at the selected value of the LFL filter, instead of the calculation group managing it centrally:
We refresh Power BI and find that the results are now as expected, the original and custom measures now match when using the calculation group as a filter: