Section 3 Control Charts
Lesson S03-05
Text: Section 3 pages 21 – 32
Duration: 23 minutes
X̄ and S Charts
The s chart can be used instead of the R chart. In fact, the standard deviation is a superior measure of variation than the range is. Ranges were used historically based on the ease of calculation. However, the standard deviation is superior since it does not “throw away” data when it’s computed.
Constructing the X̄ and s chart combination is nearly identical to the method for the X̄ and R chart combination. The only difference is in the way we estimate control limits. We will reuse the data from our X̄ and R chart discussion, and we will compare the X̄ / R pair of charts with the X̄ / s pair of charts.
Download the X̄ and S Charts Calculations Worksheet. Or view workbook page Section 3: Page 26.
Individuals and Moving Range Charts
Although sampling subgroups for control charting has many advantages, it is not always feasible.
When testing is destructive or expensive, a subgroup may be too costly. In this instance, it may only be possible to acquire a sample of size one (n = 1). The importance of rational sampling for charts of subgroups will be discussed shortly but the point here is that the inability to obtain rational samples may dictate a sample size of 1. In this case we may create individuals and moving range charts.
Here, we review only the traditional individuals and moving range charts.
Download the Individuals and MR Charts Calculations Worksheet. Or view workbook page Section 3: Page 30.
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