First a plan is formulated. This should be a plan on a limited scale to reduce risk. One factor that seems pertinent is chosen. A theory was formed to call clients the day before their appointment to remind them.
Next the plan is implemented, this is the Do stage and results in data which we Study in the next stage. Finally, based on the study we Act. We may accept and expand the plan or we may find the evidence does not support the plan. In this case it can be revised or discarded and a new plan formed.
Imperative is to act only on the evidence and not what we thought might happen or wished would happen. This is why SPC was introduced before mentioning PDSA. Each is an asset to the other.
Here’s how it worked. For ten-weeks the plan was implemented and the data recorded. Plotted on the chart results appear like this.
Question, can we say the plan accomplished its objective?
First, let me apologize for being intentionally misleading. I chose this example for a specific reason and to make a couple of specific points. The first point is, statistics can guide a manager more accurately than numbers or math alone. Second, you must understand the rules, which were not fully disclosed.
The question was, “Can we say the plan accomplished its objective?” The answer is, there is no statistical evidence to support that it has. It looks promising, but it takes seven or more points on one side of the mean to establish a trend. Six points still fall within the realm of statistical probability of mere chance. On the seventh week the no show count went to 2, crossing the mean. This ended the possibility of a trend and does not support the theory. The following week the count returned to 0 and the next [on the chart below] 3, which could have been predicted.
At this point the theory could be revised or discarded, in favor of another theory. This is an imperative step. We cannot “fall in love” with our theory and then revise the facts to support what we believed would happen. If we are to have the best likelihood of success, we must face the facts.
Company X decides to discard the theory and adopt a new one. It is decided to try explaining the importance of keeping an appointment. For nine weeks, this is done. It is explained to each caller, “Time is reserved for them and other potential clients are turned down in that time. If they no show, a technician is left without work for that time and this increases cost. It’s asked that they please phone if the appointment cannot be kept.”
The following week the plan is implemented and the results are below:
A new mean for the data is calculated and falls at 1.0. Nine consecutive points now fall at or below the mean, which is not statistically probable. This establishes a trend and supports the new theory.
At this point the theory can be accepted, more testing can be done to further prove it or another theory could be added, to see if results can be further improved.
There are several “signals” a control chart can reveal. A point outside of the control limits, high or low, is a high probability of a special cause. In this case, that might be seven or more no shows in a week. Seven or more consecutive points not crossing the mean might also establish a trend or run. Nine or more consecutive points, all rising or falling could also establish a run.
The books in the reading list, give far more detail on this topic and are well worth the read. The PDSA cycle also gives a means of improvement, greatly increasing the chances of being correct. Any number of things can be measured and improved in this manner. For example, average time of diagnosis, daily/weekly sales, comebacks, the number of rotor turns before the bits should be replaced even time of part delivery by vendors.
A special cause shows and opportunity for quick improvement and a need for quick action. Common cause variation can only be improved by improving the system. Knowing how to distinguish each can make a huge difference in the effectiveness of a manager. This knowledge is also a base on which other management tools largely depend.
Joined: 15 May 2007 Posts: 774 Location: Baton Rouge, LA
Posted: Sun Jun 03, 2007 3:58 pm Post subject:
Eric J. wrote:
Louis, this is a tad over my head. Could you post a few more examples of how you use this.
Hi Eric,
I will be glad to, and will come back to it after my next post, which will introduce a new tool. After that I will try to work in several more examples. The preferred reading list also illustrates many examples, far better than I might be able.
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