The simplest way to get a list of issues with particular custom field values would be using both dimensions Severity and Probability on Pages. There you will have an option to select Severity with value 4 and take multiple selections in Probability and select values 1,2, and 3.
If you would like to count created issues with particular values, you can use this calculated member in Mesures using both dimensions and filtering out the ones with particular values. If you address the dimension the value (name) will be represented as a string:
[Severity].CurrentMember.Name = "4"),
[Probability].CurrentMember.Name MATCHES "1|2|3"),
-- this measure in the last SUM works as a fitler and as counter
If you would like to create a calculated member in Issue dimension, then I would suggest addressing values in Severity and Probability as issue properties. Here is an example formula for this:
[Issue].CurrentHierarchyMember.GetNumber("Severity") = 4
[Issue].CurrentHierarchyMember.GetNumber("Probability") >= 3
Calculated members in Issue dimension could be easier to use in some cases. However, they could lead to some performances issues and other limitations. We suggest avoiding them, if possible and use measures or pages selections instead.
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