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.Harding at wlandres.net>
> Date: 04-Apr-2012 Time: 23:06:32
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>
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