Stacey -- your post is hardly a waste of time -- I'm glad you've contributed to the discussion. Many of your comments affirm what I've begun to understand about Wheeler's message about using process behavior charts. Indeed, he does indicate that it's hard to go wrong with plotting any type of data on an XmR, especially as a first step towards understanding the nature of a given process. And, that's essentially where I'm at -- a first step to understanding control charts, their uses, and whether these charts can be applied to the tasks that gets carried out at the company I work for.
I agree that the chart I posted certainly could use a few more data points. Even if the current chart did have a data point outside of the limits, I wouldn't necessarily feel inclined to investigate the driving forces behind this extreme data point just yet. The data plotted isn't based on an actual process, but rather the rate of compliance for patients to get a certain type of blood test done with test results falling within a desired range. So, we don't control a process, but rather influence whether patients get their appropriate blood tests done. (A lot of factors impact the outcomes of patient health care. A doctor can do all the right things, but if a patient doesn't get a test performed or doesn't get a prescription filled or doesn't change their lifestyle, then unpredictable or undesirable outcomes may likely prevail.)
As I mentioned in my previous post, many of our clinical measures (like blood test compliance) show a visible, desirable, upward shift from baseline -- i.e., once our care management team began to engage a client's patients directly or indirectly by providing consultation with physicians, the vast majority of clinical measures (both process and outcomes oriented measures) reveal an impact that is essentially evident immediately after baseline. Even with such positive results, I feel we need more data before we can presume that the positive outcomes we're witnessing early on are actually sustainable and attributable to our efforts.
Again, my purpose in initiating this discussion thread was to expose my newness to process behavior charts (SPC) and collect feedback. Hopefully along the way, other fellow readers who may be dabbling with control charts can benefit from this discussion, too. I am certainly learning quite a bit from everyone that has contributed to the discussion.
Speaking of learning, does Wheeler ever reveal how to calculate other control limits besides the upper/lower natural control limits on an XmR chart? These natural control limits reflect 3-sigma, right? So what about 2 sigma or 1 sigma limits? Perhaps Wheeler doesn't find these inner limits all that necessary or perhaps it's inappropriate to have other limits plotted on an XmR chart?