[R] Shapiro-Wilk cpoefficients: 2 Qs

[Peter Ehlers]

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
> This message was sent by XFMail
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.