SQC Graph Statistical Options
This tutorial will cover all the options available on the "Statistical Options" section on the SQC Graph window.
When all options are unchecked the SPC graph will just contain the points connected by a line
Show Average
Checking this option will add an Average Line to the chart
Average and Sample Standard Deviation value will show in the Information Box on the right side of the chart
Show Sigma 1, 2, 3
Checking any of these option will show an upper line and lower line calculated by adding or subtracting a multiple of standard deviation unit(s) from/to the average. In the calculation the standard deviation unit can be verified in excel by using formula =STDEV()
- Show Sigma 1: +/- 1 Standard Deviation From Average
- Show Sigma 2: +/-2 Standard Deviation From Average
- Show Sigma 3: +/-3 Standard Deviation From Average
The Information Box contains the Average Value, Standard Deviation Value and the values for upper and lower lines.
Show Limit 1, 2, 3
Checking any of these option will show an upper and lower specification limit for the Item. Most sites use the following Limit designations:
- Limit 1: External Specification Limits
- Limit 2: Internal Specification Limits
- Limit 3: Contractual Specification Limits (rarely used)
The Information Box contains value of each limit line and a Cp/Cpk value for each set of specifications.
Show CL
Clicking on this option will display dynamic control limits for your data set. These control limits are calculated using an average moving range method for estimating the standard deviation. This calculation matches an output in minitab for an I-Chart with method of estimating standard deviation set to "Average moving range", and a Length of moving range set to 2. These are also the control limits used to detect "Dynamic SPC failures"
The Information Box contains values of Limit Lines
Show Fixed Limits
To display Fixed Limits click first to select Show Sigma 1, 2, or 3. This will pass a multiplication factor for calculation of fixed limits. Then Select Show Fixed this will display the Upper and Lower Fixed control limit as well as the Set Average
The Information Box contains values of Fixed Limit Lines (Note letter F for fixed)
Show Target Value
When there is a target set on the Item, clicking on this check box will show it on the chart
Show Moving Range
Show moving Range Check box will display the moving range portion of the chart
Show Dropped
When investigation shows assignable cause for variation, Lot data can be dropped from SQC chart. Clicking on Show Dropped check box will allow you to see all the points that have been dropped.
Show Legends
Show Legends allows you to see a label for all the lines on your chart. It also displays some descriptive statistics about your data set.
Maximize Scale
Maximize Scale allows you to increase resolution of your chart, however this might also hide the Limits
Show Grid
Show Grid will allow you to draw grid lines on your chart
Show Values
Show Values shows a value for each data point
Show PCT Recovery
Not Used
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