R help - Oct 2002 - V-value in the wilcox.test resp. wilcox.exact

hi,
when performing a wilcox.test or a wilcox.exact i get results that looks 
like this:

wilcox.exact(x, mu=.5)
         Exact Wilcoxon signed rank test

data:  x
V = 207, p-value = 0.0006905
alternative hypothesis: true mu is not equal to 0.5


the way i understand the wilcox.test (or wilcox.exact) the V-value 
represents the summed up ranks of either the positive or negative 
differences, whichever sum is BIGGER.

to actually look up the p-value in a table (and to quote it in a 
publication) i need the T value, which to my understanding is the summed 
up ranks of either the positive or negative differences, whichever sum 
is SMALLER.

so i guess my question is: how do i get the T-value

thanks,
jan

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Jan Malte Wiener <jan.wiener at tuebingen.mpg.de> writes:
> the way i understand the wilcox.test (or wilcox.exact) the V-value
> represents the summed up ranks of either the positive or negative
> differences, whichever sum is BIGGER.
> 
> to actually look up the p-value in a table (and to quote it in a
> publication) i need the T value, which to my understanding is the
> summed up ranks of either the positive or negative differences,
> whichever sum is SMALLER.
> 
> so i guess my question is: how do i get the T-value
At least for wilcox.test, V is the sum of the *positive* ranks. The
sum of the negative ranks can be obtained by switching the sign on x
and mu. 

In the absence of ties and exact zeros, the two rank sums add up to
sum(1:N) == N*(N+1)/2

If ties are present, standard tables are wrong anyway...

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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> hi,
> when performing a wilcox.test or a wilcox.exact i get results that looks 
> like this:
> 
> wilcox.exact(x, mu=.5)
>          Exact Wilcoxon signed rank test
> 
> data:  x
> V = 207, p-value = 0.0006905
> alternative hypothesis: true mu is not equal to 0.5
> 
> 
> the way i understand the wilcox.test (or wilcox.exact) the V-value 
> represents the summed up ranks of either the positive or negative 
> differences, whichever sum is BIGGER.
no. wilcox.test computes the statistic along the following lines

        x <- x - mu
        ZEROES <- any(x == 0)
        if(ZEROES)
            x <- x[x != 0]
        ...
        r <- rank(abs(x))
        STATISTIC <- sum(r[x > 0])

i.e. the sum of the ranks of the absolute values of x which belong
to positive values of x. 
> 
> to actually look up the p-value in a table (and to quote it in a 
> publication) i need the T value, which to my understanding is the summed 
> up ranks of either the positive or negative differences, whichever sum 
> is SMALLER.
> 
wilcox.test computes the p-value for you, no need to look up tables.

Torsten
> so i guess my question is: how do i get the T-value
> 
> thanks,
> jan
> 
>
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> Send "info", "help", or "[un]subscribe"
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>
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> 
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