I and IR Subcommands (SPCHART command)

I produces an individuals chart, and IR produces an individuals chart and a moving range chart. Both types are control charts for continuous variables, such as size, weight, length, and temperature.

An individuals chart plots each individual observation on a control chart. The center line indicates the mean of all individual values, and the control limits are calculated from the mean of the moving ranges, the span, and the user-specified SIGMAS value. Individuals charts are often used with moving range charts to test process variability when the subgroup size is 1. This situation occurs frequently when automated inspection and measurement technology is used and every manufactured unit is analyzed. The situation also occurs when the process is so slow that a larger subgroup size becomes impractical. The following figure shows an individuals chart.

A moving range chart plots moving ranges of n successive observations on a control chart, where n is the specified span (see ). The center line is the mean of moving ranges, and the control limits are calculated from the ranges, the span, and the user-specified SIGMAS value (see ). The following figure shows a moving range chart.