Section 3 Control Charts
Lesson S03-10
Text: Section 3 pages 55 – 63
Duration: 15 minutes
Types of Samples
There are a multitude of statistical sampling types, but in this section, only three will be described. Of the three, only one is fundamental to the traditional control charting methods presented in this course. Let’s discuss a Random Sample, a Systematic Sample, and a Rational Sample.
Example: A Choice Between Two Plans
Option 1: Suppose you are given a choice of two sampling plans. Both plans allow you to select a sample of size 10 every two hours or thereabouts. The first strategy is a systematic plan advising you to measure one part approximately every 12 minutes. Here, you do not let the process operate too long without monitoring it.
Option 2: The second strategy is a rational sampling method advising you to measure 5 consecutively produced parts and then let the process operate for an hour or so (without looking at it), and then to select another 5 consecutively produced parts.
Which sampling plan would you choose?
The next video discusses the selection considerations.
Rational Samples
Discussion on the sample plan selection and the various considerations.
Violations of Rational Sampling
Examples include:
- Sampling from multiple locations on the same unit
- Multi-Cavity Molding Operations
- Machining Operations where multiple units are machined together
- Continuous Flow processing
Subgroups vs. Individuals
A common question is, “Why plot subgroup averages rather than individual values?” As we have seen earlier, individual measurements often do NOT follow a normal distribution. Therefore, determining appropriate control limits requires methods for nonnormal data. Averages follow a normal distribution due to the Central Limit Theorem so plotting averages is convenient.
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