As you know, to properly use control charts, you must calculate the control limits using some amount of data (usually the first 20 points) and then lock them while you track all new data. Tableau's control limits currently recalculate every time a point is added thus diminishing the ability of signaling.
Question: How do I calculate the control limits using only the first 20 data points. each data point is a month and the metric is an index.
basically I want to say " If [Date] is before August 2015 then 3*std([INDEX])" and use that as my upper and lower control limits.
P.S. I can't attach a workbook due to the sensitivity of the information