Craig H. said:
Steve:
I like how you are able to take what can be a confusing issue and make it straightforward, and in a relatively small amount of space. Nice article.
Would you expand a little on how you decided where the breaks were in the final chart?
Probably this subject would make a good article. Here is the short version, generic to any control chart:
1. Make a set of average and control limits using the first 25 points (or all points if there are less than 25).
2. If there are any statistically significant trends within the baseline:
a. If there are only a few points in the pattern, and the data appears to return to the baseline, throw those points out of the average and baseline.
b. If it looks like there is a permanent shift, end the previous baseline prior to the shift, and start a new baseline after the shift.
3. If there are statistically significant trends after the baseline time interval, and it appears to be a permanent shift, start a new baseline using the first 25 points following the shift.
4. Keep working through the data from oldest to newest until you get to the current datum.
As a criteria for "permanent shift", I use the "MW" rule - if the data make 3 changes of direction, then there are enough data for a new baseline.
This process works well for me. There are other variations though.