The service statistics widget on the dashboard.
We are tracking Sunday School as a "service" because it's more like a service than a group (has volunteers, anyone can come), but some Sundays we don't have Sunday school. In the widget I have Sundays with attendance of 10+6+7+8+5+8+6+8+3+8, which averages 7. But the widget is showing an average of 5 because we have 4 Sundays (thanksgiving, the 2 Sundays after Christmas, and the current week) with an attendance of 0.
In service reports, there's a nice button that says "Exclude services without service attendance". It would be nice if there was a similar button in the widget to "In calculating the average, exclude services without service attendance".