[R] Problem with ties in rank()

[ripley@stats.ox.ac.uk]

On Tue, 7 May 2002, arcriswell wrote:

> Hello All:
>
> I have a vector of data, z
>
> > z
>  [1] 0.1  0.1  0.1  0.1  0.2  0.2  0.3  0.3  0.3  0.4  0.5  0.5  0.5  0.7
> 0.7  0.7  0.9  0.9  1.1
> [20] 1.1  1.2  1.3  1.4
>
> The first 4 elements have values of 0.1 followed 2 elements with values 0.2.
>
> When I invoke rank(z), I expected to get (1+2+3+4)/4 = 2.5 for the first 4
> elements in the ranking and (5+6)/2 = 5.5 for elements 5 and 6. But what I
> do get is the following.
>
> > rank(z)
>  [1]  1.5  1.5  3.5  3.5  5.0  6.0  8.0  8.0  8.0 10.0 12.0 12.0 12.0
15.0 15.0
> [16] 15.0 17.5 17.5 19.5 19.5 21.0 22.0 23.0
> >
>
> Why is that???

Rounding error, I guess.

> z <- scan(n=23)
0.1  0.1  0.1  0.1  0.2  0.2  0.3  0.3  0.3  0.4  0.5  0.5  0.5  0.7
0.7  0.7  0.9  0.9  1.1  1.1  1.2  1.3  1.4

> rank(z)
 [1]  2.5  2.5  2.5  2.5  5.5  5.5  8.0  8.0  8.0 10.0 12.0 12.0 12.0 15.0 15.0
[16] 15.0 17.5 17.5 19.5 19.5 21.0 22.0 23.0
>
> I am using R 1.5.0 on Windows 2000.

So was I.  Try dput(z).

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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