qcc (version 2.2)

process.capability: Process capability analysis

Description

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

Usage

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

Arguments

object

a 'qcc' object of type "xbar"

spec.limits

a vector specifying the lower and upper specification limits.

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.

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.

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.

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.

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. (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

Examples

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))