"It is in intensity of feeling, not in statistics,
that the power to move the world lies,
though by statistics must that power be guided if it is to move the world
aright"
--- 19th century sociologist
Trend and Control Chart Interpretation
The purpose of chart interpretation is to help make better
decisions by identifying the two types of variation -- common and
special. Processes that consist of just common causes are more
predictable over time. If you want to maintain this predictability, monitor the
process and eliminate special causes when they occur. However, if you
want to improve the process, change is required. To determine if the change is
resulting in a shift in the process, an interpretation standard is needed.
The are different standards that are used for interpreting trend
and control charts. The following standard was adopted by the American Society
for Quality (ASQ) for use in their Koalaty Kid program. Koalaty Kid is the
application of the quality improvement principles, methods and tools at the K-12
level. (Reference: PQ Systems Improvement Tools, © QIP,
Inc.V-1).
Indications of special causes:
1. Run of seven points.
a. Seven points in a row above or below the center line
(average or central location).
b. Seven or more points in a row going in one
direction, up or down.
2. Any nonrandom pattern.
a. Too close to the average.
b. Too far from the average.
c. Cycles.
3. Any point lying outside the upper or lower control limits.
Generally, 20-25 data points are needed to develop upper and lower limits.
For
an applied example of this standard, see Assessing you
Quality Leadership Knowledge/Correct Answer.
Why Seven Points? If someone was to bet you
that they could flip a coin and get either seven heads or seven tails in a row,
you might take the bet because in your experience (try it with a coin), this
kind of result would be significant based on your experience with the
process. I would guess that if someone did get seven consecutive heads or
tails, you would ask to see the coin. A run of seven points is an
indication that something out of the ordinary might have changed in the
process.
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