Hi Ted,
On 2012-04-04 15:06, Ted Harding wrote:
Greetings!
I want to have the coefficients that R uses in shapiro.test()
for the Shapiro-Wilk test for a prticular sample size, i.e.
the a[i] in
W = Sum(a[i]*x[i])/(Sum(x[i] - mean(x))^2)
(where the x[i] are sorted). Two questions:
Q1:
Is there a readymade R function from which I can extract these?
Q2:
I was wondering if I might be able to modify the code for the
function shapiro.test() so as to obtain these. When I enter
shapiro.test
I get:
function (x)
{
DNAME<- deparse(substitute(x))
x<- sort(x[complete.cases(x)])
stopifnot(is.numeric(x))
n<- length(x)
if (n< 3 || n> 5000)
stop("sample size must be between 3 and 5000")
rng<- x[n] - x[1L]
if (rng == 0)
stop("all 'x' values are identical")
if (rng< 1e-10)
x<- x/rng
n2<- n%/%2L
sw<- .C(R_swilk, init = FALSE, as.single(x), n, n1 = n,
n2, a = single(n2), w = double(1), pw = double(1), ifault =
integer(1L))
if (sw$ifault&& sw$ifault != 7)
stop(gettextf("ifault=%d. This should not happen", sw$ifault),
domain = NA)
RVAL<- list(statistic = c(W = sw$w), p.value = sw$pw, method =
"Shapiro-Wilk normality test",
data.name = DNAME)
class(RVAL)<- "htest"
return(RVAL)
}
<environment: namespace:stats>
So, on the off-chance that the variable 'sw' computed therein might
contain something useful, I changed "return(RVAL)" to "return(sw)",
just in case the coefficients might be lurking as a component of sw,
and used this to define a function SW_ted(). I then ran
SW_ted(rnorm(30))
# Error in SW_ted(rnorm(30)) : object 'R_swilk' not found
Since shapiro.test(rnorm(30)) works perfectly, and since the
"stats:" namespace is already present, I am wondering why
"object 'R_swilk' not found" when it clearly can be found by
shapiro.test().
So why is it that in the ".C" call:
sw<- .C(R_swilk, ... )
my modifiction of shapiro.test() doesn't find it?
(No doubt there is some dumb oversight behind this, but I'd be
grateful to be told what it is)!
No dumb oversight that I can see, but:
Q1: I have no idea, but I doubt that there is an existing function.
Q2: I think that the environment of the newly created function
SW_ted needs to be set appropriately; try this:
environment(SW_ted) <- environment(shapiro.test)
More information should be available from the constants used in
swilk.c (in the R sources) which is a translation of the Fortran
code at http://lib.stat.cmu.edu/apstat/R94.
Peter Ehlers
With thanks,
Ted.
-------------------------------------------------
E-Mail: (Ted Harding)<ted.hard...@wlandres.net>
Date: 04-Apr-2012 Time: 23:06:32
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