xewma.arl {spc}

Compute ARLs of EWMA control charts

Description

Computation of the (zero-state) Average Run Length (ARL) for different types of EWMA control charts monitoring normal mean.

Usage

xewma.arl(l,cE,mu,zr=0,hs=0,sided="one",limits="fix",q=1,
steady.state.mode="conditional",r=40)

Arguments

l

smoothing parameter lambda of the EWMA control chart.

cE

critical value (similar to alarm limit) of the EWMA control chart.

mu

true mean.

zr

reflection border for the one-sided chart.

hs

so-called headstart (enables fast initial response).

sided

distinguishes between one- and two-sided EWMA control chart by choosing "one" and "two", respectively.

limits

distinguishes between different control limits behavior.

q

change point position. For q=1 and \mu=\mu_0 and \mu=\mu_1, the usual zero-state ARLs for the in-control and out-of-control case, respectively, are calculated. For q>1 and \mu!=0 conditional delays, that is, E_q(L-q+1|L\ge q), will be determined. Note that mu0=0 is implicitely fixed.

steady.state.mode

distinguishes between two steady-state modes – conditional and cyclical (needed for q>1).

r

number of quadrature nodes, dimension of the resulting linear equation system is equal to r+1 (one-sided) or r (two-sided).

Details

In case of the EWMA chart with fixed control limits, xewma.arl determines the Average Run Length (ARL) by numerically solving the related ARL integral equation by means of the Nystroem method based on Gauss-Legendre quadrature. If limits is not "fix", then the method presented in Knoth (2003) is utilized. Note that for one-sided EWMA charts (sided="one"), only "vacl" and "stat" are deployed, while for two-sided ones (sided="two") also "fir", "both" (combination of "fir" and "vacl"), "Steiner" and "cfar" are implemented. For details see Knoth (2004).

Value

Except for the fixed limits EWMA charts it returns a single value which resembles the ARL. For fixed limits charts, it returns a vector of length q which resembles the ARL and the sequence of conditional expected delays for q=1 and q>1, respectively.

Author(s)

Sven Knoth

References

K.-H. Waldmann (1986), Bounds for the distribution of the run length of geometric moving average charts, Appl. Statist. 35, 151-158.

S. V. Crowder (1987), A simple method for studying run-length distributions of exponentially weighted moving average charts, Technometrics 29, 401-407.

J. M. Lucas and M. S. Saccucci (1990), Exponentially weighted moving average control schemes: Properties and enhancements, Technometrics 32, 1-12.

S. Chandrasekaran, J. R. English and R. L. Disney (1995), Modeling and analysis of EWMA control schemes with variance-adjusted control limits, IIE Transactions 277, 282-290.

T. R. Rhoads, D. C. Montgomery and C. M. Mastrangelo (1996), Fast initial response scheme for exponentially weighted moving average control chart, Quality Engineering 9, 317-327.

S. H. Steiner (1999), EWMA control charts with time-varying control limits and fast initial response, Journal of Quality Technology 31, 75-86.

S. Knoth (2003), EWMA schemes with non-homogeneous transition kernels, Sequential Analysis 22, 241-255.

S. Knoth (2004), Fast initial response features for EWMA Control Charts, Statistical Papers 46, 47-64.

See Also

xcusum.arl for zero-state ARL computation of CUSUM control charts and xewma.ad for the steady-state ARL.

Examples

## Waldmann (1986), one-sided EWMA
l <- .75
round(xewma.arl(l,2*sqrt((2-l)/l),0,zr=-4*sqrt((2-l)/l)),digits=1)
l <- .5
round(xewma.arl(l,2*sqrt((2-l)/l),0,zr=-4*sqrt((2-l)/l)),digits=1)
## original values are 209.3 and 3907.5 (in Table 2).

## Waldmann (1986), two-sided EWMA with fixed control limits
l <- .75
round(xewma.arl(l,2*sqrt((2-l)/l),0,sided="two"),digits=1)
l <- .5
round(xewma.arl(l,2*sqrt((2-l)/l),0,sided="two"),digits=1)
## original values are 104.0 and 1952 (in Table 1).

## Crowder (1987), two-sided EWMA with fixed control limits
l1 <- .5
l2 <- .05
cE <- 2
mu <- (0:16)/4
arl1 <- sapply(mu,l=l1,cE=cE,sided="two",xewma.arl)
arl2 <- sapply(mu,l=l2,cE=cE,sided="two",xewma.arl)
round(cbind(mu,arl1,arl2),digits=2)

## original results are (in Table 1)
## 0.00 26.45 127.53
## 0.25 20.12  43.94
## 0.50 11.89  18.97
## 0.75  7.29  11.64
## 1.00  4.91   8.38
## 1.25  3.95*  6.56
## 1.50  2.80   5.41
## 1.75  2.29   4.62
## 2.00  1.94   4.04
## 2.25  1.70   3.61
## 2.50  1.51   3.26
## 2.75  1.37   2.99
## 3.00  1.26   2.76
## 3.25  1.18   2.56
## 3.50  1.12   2.39
## 3.75  1.08   2.26
## 4.00  1.05   2.15  (* -- in Crowder (1987) typo!?)

## Lucas/Saccucci (1990)
## two-sided EWMA

## with fixed limits
l1 <- .5
l2 <- .03
c1 <- 3.071
c2 <- 2.437
mu <- c(0,.25,.5,.75,1,1.5,2,2.5,3,3.5,4,5)
arl1 <- sapply(mu,l=l1,cE=c1,sided="two",xewma.arl)
arl2 <- sapply(mu,l=l2,cE=c2,sided="two",xewma.arl)
round(cbind(mu,arl1,arl2),digits=2)

## original results are (in Table 3)
## 0.00 500.   500.
## 0.25 255.    76.7
## 0.50  88.8   29.3
## 0.75  35.9   17.6
## 1.00  17.5   12.6
## 1.50   6.53   8.07
## 2.00   3.63   5.99
## 2.50   2.50   4.80
## 3.00   1.93   4.03
## 3.50   1.58   3.49
## 4.00   1.34   3.11
## 5.00   1.07   2.55

## Not run: 
## with fir feature
l1 <- .5
l2 <- .03
c1 <- 3.071
c2 <- 2.437
hs1 <- c1/2
hs2 <- c2/2
mu <- c(0,.5,1,2,3,5)
arl1 <- sapply(mu,l=l1,cE=c1,hs=hs1,sided="two",limits="fir",xewma.arl)
arl2 <- sapply(mu,l=l2,cE=c2,hs=hs2,sided="two",limits="fir",xewma.arl)
round(cbind(mu,arl1,arl2),digits=2)

## original results are (in Table 5)
## 0.0 487.   406.
## 0.5  86.1   18.4
## 1.0  15.9    7.36
## 2.0   2.87   3.43
## 3.0   1.45   2.34
## 5.0   1.01   1.57

## Chandrasekaran, English, Disney (1995)
## two-sided EWMA with fixed and variance adjusted limits (vacl)

l1 <- .25
l2 <- .1
c1s <- 2.9985
c1n <- 3.0042
c2s <- 2.8159
c2n <- 2.8452
mu <- c(0,.25,.5,.75,1,2)
arl1s <- sapply(mu,l=l1,cE=c1s,sided="two",xewma.arl)
arl1n <- sapply(mu,l=l1,cE=c1n,sided="two",limits="vacl",xewma.arl)
arl2s <- sapply(mu,l=l2,cE=c2s,sided="two",xewma.arl)
arl2n <- sapply(mu,l=l2,cE=c2n,sided="two",limits="vacl",xewma.arl)
round(cbind(mu,arl1s,arl1n,arl2s,arl2n),digits=2)

## original results are (in Table 2)
## 0.00 500.   500.   500.   500.
## 0.25 170.09 167.54 105.90  96.6
## 0.50  48.14  45.65  31.08  24.35
## 0.75  20.02  19.72  15.71  10.74
## 1.00  11.07   9.37  10.23   6.35
## 2.00   3.59   2.64   4.32   2.73

## The results in Chandrasekaran, English, Disney (1995) are not
## that accurate. Let us consider the more appropriate comparison

c1s <- xewma.crit(l1,500,sided="two")
c1n <- xewma.crit(l1,500,sided="two",limits="vacl")
c2s <- xewma.crit(l2,500,sided="two")
c2n <- xewma.crit(l2,500,sided="two",limits="vacl")
mu <- c(0,.25,.5,.75,1,2)
arl1s <- sapply(mu,l=l1,cE=c1s,sided="two",xewma.arl)
arl1n <- sapply(mu,l=l1,cE=c1n,sided="two",limits="vacl",xewma.arl)
arl2s <- sapply(mu,l=l2,cE=c2s,sided="two",xewma.arl)
arl2n <- sapply(mu,l=l2,cE=c2n,sided="two",limits="vacl",xewma.arl)
round(cbind(mu,arl1s,arl1n,arl2s,arl2n),digits=2)

## which demonstrate the abilities of the variance-adjusted limits
## scheme more explicitely.

## Rhoads, Montgomery, Mastrangelo (1996)
## two-sided EWMA with fixed and variance adjusted limits (vacl),
## with fir and both features

l <- .03
cE <- 2.437
mu <- c(0,.5,1,1.5,2,3,4)
sl <- sqrt(l*(2-l))
arlfix  <- sapply(mu,l=l,cE=cE,sided="two",xewma.arl)
arlvacl <- sapply(mu,l=l,cE=cE,sided="two",limits="vacl",xewma.arl)
arlfir  <- sapply(mu,l=l,cE=cE,hs=c/2,sided="two",limits="fir",xewma.arl)
arlboth <- sapply(mu,l=l,cE=cE,hs=c/2*sl,sided="two",limits="both",xewma.arl)
round(cbind(mu,arlfix,arlvacl,arlfir,arlboth),digits=1)

## original results are (in Table 1)
## 0.0 477.3* 427.9* 383.4* 286.2*
## 0.5  29.7   20.0   18.6   12.8
## 1.0  12.5    6.5    7.4    3.6
## 1.5   8.1    3.3    4.6    1.9
## 2.0   6.0    2.2    3.4    1.4
## 3.0   4.0    1.3    2.4    1.0
## 4.0   3.1    1.1    1.9    1.0
## * -- the in-control values differ sustainably from the true values!

## Steiner (1999)
## two-sided EWMA control charts with various modifications

## fixed vs. variance adjusted limits

l <- .05
cE <- 3
mu <- c(0,.25,.5,.75,1,1.5,2,2.5,3,3.5,4)
arlfix <- sapply(mu,l=l,cE=cE,sided="two",xewma.arl)
arlvacl <- sapply(mu,l=l,cE=cE,sided="two",limits="vacl",xewma.arl)
round(cbind(mu,arlfix,arlvacl),digits=1)

## original results are (in Table 2)
## 0.00 1379.0   1353.0
## 0.25  135.0    127.0
## 0.50   37.4     32.5 
## 0.75   20.0     15.6
## 1.00   13.5      9.0
## 1.50    8.3      4.5
## 2.00    6.0      2.8
## 2.50    4.8      2.0
## 3.00    4.0      1.6
## 3.50    3.4      1.3
## 4.00    3.0      1.1

## fir, both, and Steiner's modification

l <- .03
cfir <- 2.44
cboth <- 2.54
cstein <- 2.55
hsfir <- cfir/2
hsboth <- cboth/2*sqrt(l*(2-l))
mu <- c(0,.5,1,1.5,2,3,4)
arlfir <- sapply(mu,l=l,cE=cfir,hs=hsfir,sided="two",limits="fir",xewma.arl)
arlboth <- sapply(mu,l=l,cE=cboth,hs=hsboth,sided="two",limits="both",xewma.arl)
arlstein <- sapply(mu,l=l,cE=cstein,sided="two",limits="Steiner",xewma.arl)
round(cbind(mu,arlfir,arlboth,arlstein),digits=1)

## original values are (in Table 5)
## 0.0 383.0   384.0   391.0
## 0.5  18.6    14.9    13.8
## 1.0   7.4     3.9     3.6
## 1.5   4.6     2.0     1.8
## 2.0   3.4     1.4     1.3
## 3.0   2.4     1.1     1.0
## 4.0   1.9     1.0     1.0

## SAS/QC manual 1999
## two-sided EWMA control charts with fixed limits

l <- .25
cE <- 3
mu <- 1
print(xewma.arl(l,cE,mu,sided="two"),digits=11)

# original value is 11.154267016.

## Some recent examples for one-sided EWMA charts
## with varying limits and in the so-called stationary mode

# 1. varying limits = "vacl"

lambda <- .1
L0 <- 500

## Monte Carlo results (10^9 replicates)
# mu    ARL      s.e.
# 0     500.00   0.0160
# 0.5   21.637   0.0006
# 1     6.7596   0.0001
# 1.5   3.5398   0.0001
# 2     2.3038   0.0000
# 2.5   1.7004   0.0000
# 3     1.3675   0.0000

zr <- -6
r <- 50
cE <- xewma.crit(lambda, L0, zr=zr, limits="vacl", r=r)
Mxewma.arl <- Vectorize(xewma.arl, "mu")
mus <- (0:6)/2
arls <- round(Mxewma.arl(lambda, cE, mus, zr=zr, limits="vacl", r=r), digits=4)
data.frame(mus, arls)

# 2. stationary mode, i. e. limits = "stat"

## Monte Carlo results (10^9 replicates)
# mu    ARL      s.e.
# 0     500.00   0.0159
# 0.5   22.313   0.0006
# 1     7.2920   0.0001
# 1.5   3.9064   0.0001
# 2     2.5131   0.0000
# 2.5   1.7983   0.0000
# 3     1.4029   0.0000

cE <- xewma.crit(lambda, L0, zr=zr, limits="stat", r=r)
arls <- round(Mxewma.arl(lambda, cE, mus, zr=zr, limits="stat", r=r), digits=4)
data.frame(mus, arls)

## End(Not run)