stats.g: Statistics used in computing and drawing a Shewhart g chart

In luca-scr/qcc: Quality Control Charts

Statistics used in computing and drawing a Shewhart g chart

Description

These functions are used to compute statistics required by the g chart (geometric distribution) for use with the qcc package.

Usage

stats.g(data, sizes)
sd.g(data, sizes, ...)
limits.g(center, std.dev, sizes, nsigmas = NULL, conf = NULL)

Arguments

data	the observed data values
center	sample center statistic
sizes	sample sizes (not used)
std.dev	standard deviation of geometric distribution
nsigmas	a numeric value specifying the number of sigmas to use for computing control limits. It is ignored when the conf argument is provided.
conf	a numeric value in (0,1) specifying the confidence level to use for computing control limits.
...	catches further ignored arguments.

Details

The g chart plots the number of non-events between events. np charts do not work well when the probability of an event is rare (see example below). Instead of plotting the number of events, the g chart plots the number of non-events between events.

Value

The function stats.g() returns a list with components statistics and center.

The function sd.g() returns std.dev the standard deviation sqrt(1-p)/p.

The function limits.g() returns a matrix with lower and upper control limits.

Note

The geometric distribution is quite skewed so it is best to set conf at the required confidence interval (0 < conf < 1) rather than as a multiplier of sigma.

Author(s)

Greg Snow [e-mail]

References

Kaminsky, FC et. al. (1992) Statistical Control Charts Based on a Geometric Distribution, Journal of Quality Technology, 24, pp 63–69.

Yang, Z et. al. (2002) On the Performance of Geometric Charts with Estimated Control Limits, Journal of Quality Technology, 34, pp 448–458.

See Also

qcc

Examples

success  = rbinom(1000, 1, 0.01)
num.noevent  = diff(which(c(1,success)==1))-1
qcc(success, type = "np", sizes = 1)
qcc(num.noevent, type = "g")

luca-scr/qcc documentation built on June 14, 2025, 9:55 p.m.