Last modified: 2013-05-02 18:12:46 UTC
See example URL. Σ column is never calculated because there's always some column with a missing value. Unless all runs of the update script are very unlucky, I suppose this is because when there are 0 users in the required activity range the value is not shown as 0 but as null (missing), which then confuses the Σ calculation.
-> NEW
[mass-moving wikistats reports from Wikimedia→Statistics to Analytics→Wikistats to have stats issues under one Bugzilla product (see bug 42088) - sorry for the bugspam!]
It seems this was fixed at some point.