schart (Statistics Toolbox)

Chart of standard deviation for Statistical Process Control.

Syntax

schart(DATA,conf)
schart(DATA,conf,specs)
schart(DATA,conf,specs)
[outliers,h] = schart(DATA,conf,specs)

Description

schart(data) displays an S chart of the grouped responses in DATA. The rows of DATA contain replicate observations taken at a given time. The rows must be in time order. The upper and lower control limits are a 99% confidence interval on a new observation from the process. So, roughly 99% of the plotted points should fall between the control limits.

schart(DATA,conf) allows control of the the confidence level of the upper and lower plotted confidence limits. For example, conf = 0.95 plots 95% confidence intervals.

schart(DATA,conf,specs) plots the specification limits in the two element vector, specs.

[outliers,h] = schart(data,conf,specs) returns outliers, a vector of indices to the rows where the mean of DATA is out of control, and h, a vector of handles to the plotted lines.

Example

This example plots an S chart of measurements on newly machined parts, taken at one hour intervals for 36 hours. Each row of the runout matrix contains the measurements for 4 parts chosen at random. The values indicate, in thousandths of an inch, the amount the part radius differs from the target radius.

load parts
schart(runout)

Reference

Montgomery, D., Introduction to Statistical Quality Control, John Wiley and Sons 1991. p. 235.