Calculating Mean Time Between Failures with multiple data groups

I am trying to calculate the Avg time between failures and average it accross groups.  The formula below works for 1 group (System ID) but does not work for 2 or more.  I have included the data and a sample DXP.  I am looking for suggestions to the forumla or approach. 

Thanks for the time and effort.



Avg(DateDiff("day",First([MTBF Service Call Start Date]) OVER (Intersect(All([Axis.X]),[System ID],Previous([MTBF Service Call Start Date]))),[MTBF Service Call Start Date])) AS [MTBF], UniqueCount([Service Call Number]) AS [SR Cnt]





Binary Data mtbf_issue.dxp177.65 KB
Office spreadsheet icon mtbf_issue.xls37.5 KB

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