Section 3 Control Charts
Lesson S03-03
Text: Section 3 pages 17 – 20
Duration: 11 minutes
The Purpose of Charts
Let’s start with a couple of questions.
Does the process appear to be in control or out? View the next video for answers/discussion.
Is the Process Stable Discussion
Does the process produce units that meet specification consistently? Will the process produce units in spec at least 95% of the time? Click the Discussion button for a short note. View the next video for answers/discussion.
The variability observed on the X̄ chart is the variability among averages. Specification limits apply to individual measurements, not averages. Customers experience the variability among individuals, but that is not represented on the control chart!
Meeting Specifications Discussion
Keep in mind that the standard deviation of the averages (standard error) is smaller than the standard deviation of the individuals.
$$ \displaystyle\large {{\sigma }_{{\bar{x}}}}=\frac{\sigma }{\sqrt{n}}$$
Click the Discussion button for a short note. View the next video for answers/discussion.
1. Averages follow a known distribution (Normal) and individuals do not generally follow an easily described distribution. 2. Averages are much more sensitive to detecting process shifts than individual values are, and the purpose is to detect shifts when they occur! This point will be discussed in detail later in this section.
How about Plotting Individual Values?
There are some tradeoffs between plotting averages versus individuals where averages provide some distinct advantages.
One of the Most Important Points!
The purpose of control charting is to learn when the process
has done something unexpected (and that does not mean “out of
spec”). With that information, we can quickly learn how the process
is affected by various factors potentially affecting it.
The purpose of control charting is NOT to identify parts out of
print (specification). In fact, since X̄ charts monitor averages,
they do not indicate whether parts are out of tolerance.
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