Overview (SPCHART command)

SPCHART generates several types of high-resolution control charts. A control chart plots a quality characteristic that is measured or computed from a sample versus the sample number or time. This technique is a widely used process-control technique for testing the hypothesis that the process is in a state of statistical control. All control charts display four series:

  • The process line representing the quality characteristic for each sample.
  • The center line indicating the average value of the quality characteristic that corresponds to the in-control state.
  • Two horizontal lines showing the upper control limit and lower control limit.

Control charts are used for improving productivity, preventing defects and unnecessary process adjustments, and gathering information about process capability.

SPCHART produces X-bar, R, s, individuals, and moving range charts as well as np, p, c, and u charts. It also produces a process capability chart. You may need to transform your data to conform to the required data organization that is described under each chart type subcommand.

Control charts are available only on systems where high-resolution display is available.

Options

Titles and Footnotes. You can use the TITLE, SUBTITLE, and FOOTNOTE subcommands to specify a title, subtitle, and footnote for the chart.

Chart Type. You can request a specific type of chart by using the XR, XS, IR, I, NP, P, C, U, or CPCHART subcommand.

Templates. You can specify a template, using the TEMPLATE subcommand, to override the default chart attribute settings on your system.

Control Limits. You can specify a sigma value on the SIGMAS subcommand to modify the calculated upper and lower control limits. You can also use the USL and LSL subcommands to specify fixed limits. The upper and lower limits that you specify will be displayed simultaneously with the calculated control limits. Note: Calculated control limits are not displayed on process capability charts.

Control Rules. You can specify control rules that help you quickly identify out-of-control points.

Basic Specification

The basic specification is a chart type subcommand with a variable specification that describes the process data.

Subcommand Order

Subcommands can be specified in any order.

Syntax Rules

  • Only one chart type subcommand can be specified.
  • Keyword SPAN is used only with IR and I subcommands.
  • Keyword CONFORM or NONCONFORM is used only with NP and P subcommands.
  • Keywords NORMAL and REFERENCE are used only with the CPCHART subcommand.
  • Keywords LSL and USL are required when the CPCHART subcommand is specified.

Operations

  • All control charts generated by SPCHART contain four basic series: the process, the center line, the upper control line, and the lower control line.
  • The chart title, subtitle, and footnote are assigned as they are specified on TITLE, SUBTITLE, and FOOTNOTE subcommands. For control charts, the default chart title is Control Chart, followed by the label of the process variable, and a one-line default footnote displays the sigma level. For a process capability chart, the default chart title is Process Capability and there is no default footnote. For all chart types, if split-file processing is in effect then a default subtitle is added that provides split-file information.
  • For control charts, the category variable label is used as the title for the category axis. If no variable label is defined, the variable name is used. If no category variable is defined, the title is null.
  • For control charts, the category variable value labels are used as the category axis labels. If no value labels are defined, values are used. If no category variable is defined, integer values from 1 to n are used, where n is the number of subgroups or units plotted.
  • For control charts, all series are plotted as lines. When a series has a constant value across all samples, the value is reported in the legend entry for the series.
  • For all chart types, case weights are not honored when each case is a subgroup. Case weights are honored when each case is a unit and when the weights are integers. When weighted data are used in an individuals chart, replicated cases are plotted on the control chart.
  • For control charts, the calculated control limits are always displayed and can be suppressed only by editing the chart in a chart window.
  • You can specify preset control limits for an X-bar or I chart, as some industries often do. The specified control limits are displayed simultaneously with the calculated limits.

Limitations

  • Control charts cannot have fewer than 2 points or more than 3000 points.
  • The subgroup size in X-bar and range charts cannot exceed 100.
  • The span for individuals charts is limited to 100.
  • For process capability charts, the subgroup size cannot exceed 100 when /CAPSIGMA=RBAR, and the span cannot exceed 100 when /CAPSIGMA=MRBAR. In addition, the subgroup size or span cannot be less than 2.