Quality Control Charts
SigmaPlot can produce extremely high quality graphs for quality control although it is perhaps overkill for it to be used as statistical quality control program. The Reference Lines feature is used to create X-bar, Range and Sigma charts. Also the HistGaus.xfm transform provided with SigmaPlot allows you to create one type of Process Capability chart.
Consider daily triplicate measurements of plate reader fluorescence for a fixed standard concentration over a months time period.
A segment of the data is shown in Figure 1. The rowwise statistics transform was used to generate the mean standard deviation (sigma) and range statistics for each group.
Figure 1. Daily triplicate measurements of the plate reader fluorescence standard. A transform created the rowwise statistics starting in column 6.
For an Xbar – Range chart, line & scatter plots are created for the group mean and group range from columns 1, 6 and 9. Then the Reference Lines option from the Plot tab of the Graph Properties dialog is used to specify the control line options.
The options selected for the Xbar chart are shown in Figure 2. Upper and lower control lines (UCL, LCL), upper and lower specification lines (U Spec, L Spec) and a mean line are selected. The UCL was specified to be located at the mean + 2 standard deviations of the data. Standard error or 95% or 99% confidence interval could also have been selected from the dropdown list and the user can enter a multiplier for, in this case, the number of standard deviations.
The “x mean only” option is used for the mean line. The “k constant” option is used for the specification lines – in this case U Spec = 25,400 and L Spec = 24,600. Other options in this dialog specify the control line title and its location, front/back placement of the control lines with respect to other graph objects and horizontal/vertical orientation of the control lines.
Figure 2. The Reference Line dialog for the Xbar chart.
The Xbar – R chart is a vertical concatenation of the Xbar and Range charts. This is shown in Figure 3 for the data in Figure 1.
Figure 3. Xbar – R chart using Reference Line options to create the control, mean and specification lines.
Download the file xbar-r_ chart.zip and double click on it to obtain the SigmaPlot notebook with the data and graph for this example.