09. SPC – Quantitative Control Chart
SPC control chart determines the current or improved processes’ control state while evaluating data point trend, and the control chart can be defined as either quantitative (continuous) or attribute (discrete) control chart based on the data you collected. In this section, quantitative control charts will be discussed and the following are the general types of quantitative control charts with respective description. And for the quantitative chart selection, please refer to the SPC introduction link for SPC general introduction.
The following table summarizes the common quantitative control charts.
Detailed Quantitative Control Chart explanation
Quantitative Control Chart Setup Steps
Quantitative Control Chart’s Limit Calculation
And for the quantitative control charts, there are constant charts for the respective coefficient to calculate the upper and lower control limits.
For the SPC Control Chart Constant Reference, check the ORIGINAL WEBSITE LINK for detailed value and the toggle below for each table’s constant application.
Limits based on subgroup’s standard deviation (s)
X-bar (X’) limits based on subgroup range.
Range Limits
X’ limits based on range chart
X’ limits based on standard deviation (s).
LCL constant for s chart
UCL constant for s chart
LCL constant for R chart
UCL constant for R chart
The following examples are listed as the example for quantitative control charts.
X-Bar R Example:
This example indicates component’s output variable (30 subgroups with 9 observations per subgroup). The range and the averages for the sample subgroups indicate the process are in a stable control and not exceeding the control limit.
X-Bar s Example:
This example indicates component’s output variable (30 subgroups with 15 observations per subgroup). The range and the averages for the sample subgroups indicate the process have some slight variance since one subgroup is exceeding the control limited. Therefore the out of control batch is required to be investigated.
X-MR Example:
This example indicates component’s output variable (30 subgroups with 1 measurement per subgroup). The moving range and the measurement for the sample subgroups indicate the process are in a stable control and not exceeding the control limit.
