Accessing control chart results in R? - r

Question

I have a short R script that loads a bunch of data and plots it in an XBar chart. Using the following code, I can plot the data and view the various statistical information.
library(qcc)
tir<-read.table("data.dat", header=T,,sep="\t")
names(tir)
attach(tir)
rand <- sample(tir)
xbarchart <- qcc(rand[1:100,],type="R")
summary(xbarchart)
I want to be able to do some process capability analysis (described here(PDF) on page 5) immediately after the XBar chart is created. In order to create the analysis chart, I need to store the LCL and UCL results from the XBar chart results created before as variables. Is there any way I can do this?

Answer 1

I shall answer your question using the example in the ?qcc help file.
x <- c(33.75, 33.05, 34, 33.81, 33.46, 34.02, 33.68, 33.27, 33.49, 33.20,
33.62, 33.00, 33.54, 33.12, 33.84)
xbarchart <- qcc(x, type="xbar.one", std.dev = "SD")
A useful function to inspect the structure of variables and function results is str(), short for structure.
str(xbarchart)
List of 11
$ call : language qcc(data = x, type = "xbar.one", std.dev = "SD")
$ type : chr "xbar.one"
$ data.name : chr "x"
$ data : num [1:15, 1] 33.8 33 34 33.8 33.5 ...
..- attr(*, "dimnames")=List of 2
.. ..$ Group : chr [1:15] "1" "2" "3" "4" ...
.. ..$ Samples: NULL
$ statistics: Named num [1:15] 33.8 33 34 33.8 33.5 ...
..- attr(*, "names")= chr [1:15] "1" "2" "3" "4" ...
$ sizes : int [1:15] 1 1 1 1 1 1 1 1 1 1 ...
$ center : num 33.5
$ std.dev : num 0.342
$ nsigmas : num 3
$ limits : num [1, 1:2] 32.5 34.5
..- attr(*, "dimnames")=List of 2
.. ..$ : chr ""
.. ..$ : chr [1:2] "LCL" "UCL"
$ violations:List of 2
..$ beyond.limits : int(0)
..$ violating.runs: num(0)
- attr(*, "class")= chr "qcc"
You will notice the second to last element in this list is called $limits and contains the two values for LCL and UCL.
It is simple to extract this element:
limits <- xbarchart$limits
limits
LCL UCL
32.49855 34.54811
Thus LCL <- limits[1] and UCL <- limits[2]