R: Functions to plot Shewhart xbar chart

stats.xbar {qcc}

R Documentation

Functions to plot Shewhart xbar chart

Description

These functions are used to compute statistics required by the xbar chart.

Usage

stats.xbar(data, sizes)
sd.xbar(data, sizes, std.dev, equal.sd = TRUE)
limits.xbar(center, std.dev, sizes, conf)

Arguments

`data`	the observed data values
`center`	sample/group center statistic.
`sizes`	samples sizes. Optional
`std.dev`	within group standard deviation. Optional for `sd.xbar` function, required for `limits.xbar`
`equal.sd`	a logical indicating whether the standard deviation is constant for all samples/groups.
`conf`	a numeric value used to compute control limits, specifying the number of standard deviations (if `conf` > 1) or the confidence level (if 0 < `conf` < 1).

Value

The function stats.xbar returns a list with components statistics and center.
The function sd.xbar returns std.dev the standard deviation of the statistic charted. This is based on results from Burr (1969).
The function limits.xbar returns a matrix with lower and upper control limits.

Author(s)

Luca Scrucca luca@stat.unipg.it

References

Burr, I.W. (1969) Control charts for measurements with varying sample sizes. Journal of Quality Technology, 1(3), 163-167.
Montgomery, D.C. (2000) Introduction to Statistical Quality Control, 4th ed. New York: John Wiley & Sons.
Wetherill, G.B. and Brown, D.W. (1991) Statistical Process Control. New York: Chapman & Hall.