qcc (version 1.2)

ewma.qcc: EWMA chart

Description

Draw an EWMA chart from an object of class `qcc'.

Usage

## S3 method for class 'qcc':
ewma(object, lambda=0.2, nsigmas=object$nsigmas, 
     add.stats = TRUE, xlab, ylab, ylim = NULL, axes.las = 0, 
     restore.par = TRUE, \dots)

Arguments

object

an object of class `qcc'.

lambda

the smoothing parameter $0 \le \lambda \le 1$

nsigmas

a numeric value specifying th number of sigmas to use for computing control limits.

add.stats

a logical value indicating whether statistics and other information should be printed at the bottom of the chart.

xlab

a string giving the label for the x-axis.

ylab

a string giving the label for the y-axis.

ylim

a numeric vector specifying the limits for the y-axis.

axes.las

numeric in {0,1,2,3} specifying the style of axis labels. See help(par).

restore.par

a logical value indicating whether the previous par settings must be restored. If you need to add points, lines, etc. to a control chart set this to FALSE.

...

Value

• Returns an object of class `ewma' which inherits from the `qcc' object. No methods are specifically defined for the `ewma' class.

Details

EWMA chart smooths a series of data based on a moving average with weights which decay exponentially. Useful to detect small and permanent variation on the mean of the process.

References

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.

See Also

qcc, ewmaSmooth, cusum

Examples

data(pistonrings)
attach(pistonrings)
diameter <- qcc.groups(diameter, sample)
q <- qcc(diameter[1:25,], type="xbar", nsigmas=3, plot=FALSE)
ewma(q, lambda=0.2)
q <- qcc(diameter[1:25,], newdata=diameter[26:40,], type="xbar", plot=FALSE)
ewma(q, lambda=0.2, nsigmas=2.7)