Process capability analysis

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Description

Computes process capability indices for a 'qcc' object of type "xbar" and plot the histogram.

Usage

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process.capability(object,spec.limits, target, std.dev, nsigmas, 
                   confidence.level = 0.95, breaks = "scott", 
                   add.stats = TRUE, print = TRUE, 
                   digits = getOption("digits"), restore.par = TRUE)

Arguments

object

a 'qcc' object of type "xbar"

spec.limits

a two-values vector specifying the lower and upper specification limits. For one-sided specification limits, the value of the missing limit must be set to NA.

target

a value specifying the target of the process. If missing the value from the 'qcc' object is used if not NULL, otherwise the target is set at the middle value bewteen specification limits.

std.dev

a value specifying the within-group standard deviation. If not provided is taken from the 'qcc' object.

nsigmas

a numeric value specifying the number of sigmas to use. If not provided is taken from the 'qcc' object.

confidence.level

a numeric value between 0 and 1 specifying the level to use for computing confidence intervals.

breaks

a value or string used to draw the histogram. See the help for hist for more details.

add.stats

a logical value indicating whether statistics and capability indices should be added at the bottom of the chart.

print

a logical value indicating whether statistics and capability indices should be printed.

digits

the number of significant digits to use.

restore.par

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

Details

This function calculates confidence limits for C_p using the method described by Chou et al. (1990). Approximate confidence limits for C_pl, C_pu and C_pk are computed using the method in Bissell (1990). Confidence limits for C_pm are based on the method of Boyles (1991); this method is approximate and it assumes that the target is midway between the specification limits.

Value

Invisibly returns a list with components:

nobs

number of obserations

center

center

std.dev

standard deviation

target

target

spec.limits

a vector of values giving the lower specification limit (LSL) and the upper specification limit (USL)

indices

a matrix of capability indices (C_p, C_pl, C_pu, C_pk, C_pm) and the corresponding confindence limits.

exp

a vector of values giving the expected fraction, based on a normal approximation, of the observations less than LSL and greater than USL.

obs

a vector of values giving the fraction of observations less than LSL and greater than USL.

Author(s)

Luca Scrucca luca@stat.unipg.it

References

Bissell, A.F. (1990) How reliable is your capability index?, Applied Statistics, 39, 331-340.
Boyles, R.A. (1991) The Taguchi capability index, Journal of Quality Technology, 23, 107-126.
Chou, Y., Owen D.B. and Borrego S.A. (1990) Lower Confidence Limits on Process Capability Indices, Journal of Quality Technology, 22, 223-229.
Montgomery, D.C. (2005) Introduction to Statistical Quality Control, 5th ed. New York: John Wiley & Sons.
Wetherill, G.B. and Brown, D.W. (1991) Statistical Process Control. New York: Chapman & Hall.

See Also

qcc

Examples

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data(pistonrings)
attach(pistonrings)
diameter <- qcc.groups(diameter, sample)
q <- qcc(diameter[1:25,], type="xbar", nsigmas=3, plot=FALSE)
process.capability(q, spec.limits=c(73.95,74.05))
process.capability(q, spec.limits=c(73.95,74.05), target=74.02)
process.capability(q, spec.limits=c(73.99,74.01))
process.capability(q, spec.limits = c(73.99, 74.1))

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