[R] qcc

[Leif Kirschenbaum]

	If you examine the code for the function "violating.runs" in the qcc package (try "violating.runs") you can deconstruct it to find that the code flags runs where there are .qcc.options$run.length or more points in a row on one side of the center (process mean).
	However, the classic Shewhart rules dictate that a run of monotonically increasing or decreasing points of <run length> is also a run violation. I include here my modified version of the function "violating.runs" which also checks for these violations:

##
## Correct some typos in violating.runs from qcc package
## Added test for run.length of points monotonically increasing or decreasing
## The simplest way is to re-run the code but with "diffs"
## representing the sign of the difference from one point to the next
##
violating.runs<-function (object, run.length = qcc.options("run.length")) 
{
    center <- object$center
    statistics <- c(object$statistics, object$new.statistics)
    cl <- object$limits
    violators <- numeric()

   for(i in 1:2){
    diffs <- statistics - center
    if(i==2) {
     diffs <- c(0,diff(statistics))
	## need to decrement run.length since we're looking at differences between points
     run.length<-run.length-1
    }
    diffs[diffs > 0] <- 1
    diffs[diffs < 0] <- -1
    runs <- rle(diffs)
    index.lengths <- (1:length(runs$lengths))[runs$lengths >= run.length]
    index.stats <- 1:length(statistics)
    vruns <- rep(runs$lengths >= run.length, runs$lengths)
    vruns.above <- (vruns & (diffs > 0))
    vruns.below <- (vruns & (diffs < 0))
    rvruns.above <- rle(vruns.above)
    rvruns.below <- rle(vruns.below)
    vbeg.above <- cumsum(rvruns.above$lengths)[rvruns.above$values]-(rvruns.above$lengths - run.length)[rvruns.above$values]
    vend.above <- cumsum(rvruns.above$lengths)[rvruns.above$values]
    vbeg.below <- cumsum(rvruns.below$lengths)[rvruns.below$values]-(rvruns.below$lengths - run.length)[rvruns.below$values]
    vend.below <- cumsum(rvruns.below$lengths)[rvruns.below$values]
    if (length(vbeg.above)) {
        for (i in 1:length(vbeg.above)) violators <- c(violators, vbeg.above[i]:vend.above[i])
    }
    if (length(vbeg.below)) {
        for (i in 1:length(vbeg.below)) violators <- c(violators, vbeg.below[i]:vend.below[i])
    }
   } ## ENDOF for i in 1:2
    return(violators)
}


> 3)Is there any more criterias made somewhere ?
	The other criteria is found by the function "beyond.limits" which checks to see if any points are beyond the upper or lower control limits.

	I believe that Prof. Scrucca wrote the qcc package as a tool to demonstrate process control to his students taking his statistics classes. Hence the statistics are excellent. For example note the use of log-gamma and exponential functions in the function "sd.xbar" to calculate the constant "c4", which avoids the divison of extremely large [~ 1E20] numbers:
	c4 <- function(n) sqrt(2/(n-1)) * exp(lgamma(n/2) - lgamma((n-1)/2)))

where a textbook might write it as:
	c4 <- function(n) sqrt(2/(n-1)) * (gamma(n/2) / gamma((n-1)/2)))
	Thus I suggest that although qcc provides an excellent basis for constructing statistical control reports, that you should be prepared to modify it for your purposes (as I am heavily doing).

P.S. I change .qcc.options using syntax such as ".qcc.options$violating.runs<-7".

Leif S. Kirschenbaum, Ph.D.
Senior Yield Engineer
Reflectivity, Inc.
408-737-8100 x307
408-737-8153 (Fax)


> Date: Tue, 29 Nov 2005 02:08:57 +0200
> From: Tommi Viitanen <totavi at utu.fi>
> Subject: [R] qcc
> 
> violating.runs
[deleted]
> 
> that the criteria for the violating is 5 but
> 1)I cannot find "5" in the code of the function. Where is the "5" ?
> 2)What is the easiest way to change it ?
> 3)Is there any more criterias made somewhere ?
> 
> Yours sincerelly, Tommi Viitanen
> 
[deleted]
> ------------------------------
> 
> Date: Tue, 29 Nov 2005 08:58:24 +0100
> From: Uwe Ligges <ligges at statistik.uni-dortmund.de>
> Subject: Re: [R] qcc
> To: Tommi Viitanen <totavi at utu.fi>
> 
> 
> See ?violating.runs:
> violating.runs(object, run.length = qcc.options("run.length"))
>                                      ^^^^^ ^^^ ^^^ ^^ ^^^ ^^^
> 
[deleted]
> 
> Uwe Ligges