[R] Different behavior of median and mean function - Why?

[Christoph Jäckel]

Hi together,

below is a small example which produces outcome I do not understand,
namely that the median function works fine on a data.frame without
negative numbers, but doesn't work on a data.frame with one negative
number. I'm sure there is a reasonable explanation for that or better,
that I'm doing something wrong and someone could guide me how to solve
it. I tried googling it, but couldn't find a solution:

> #Set up data frame
> df <- data.frame(V1=c(1,2,3,4),V2=c(2,3,4,5))
> #Both work fine
> mean(df)
 V1  V2
2.5 3.5
> median(df)
[1] 2.5 3.5
>
> #Now, I just make one number negative in the data.frame
> df <- data.frame(V1=c(1,2,3,-4),V2=c(2,3,4,5))
> mean(df)#Works fine
 V1  V2
0.5 3.5
> median(df)#Why do I  get that error?
[1]  NA 0.5
Warnmeldung:
In mean.default(X[[1L]], ...) :
  argument is not numeric or logical: returning NA
> #It works fine on both columns seperately
> median(df$V1)
[1] 1.5
> median(df$V2)
[1] 3.5
>
> sessionInfo()
R version 2.13.0 (2011-04-13)
Platform: x86_64-pc-mingw32/x64 (64-bit)

locale:
[1] C

attached base packages:
[1] graphics  grDevices datasets  stats     utils     methods   base

other attached packages:
[1] urca_1.2-5      zoo_1.6-5       svSocket_0.9-51

loaded via a namespace (and not attached):
[1] grid_2.13.0     lattice_0.19-23 svMisc_0.9-61   tcltk_2.13.0
tools_2.13.0


Thanks Christoph
--
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Christoph Jäckel (Dipl.-Kfm.)

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Research Assistant

Chair for Financial Management and Capital Markets | Lehrstuhl für
Finanzmanagement und Kapitalmärkte

TUM School of Management | Technische Universität München

Arcisstr. 21 | D-80333 München | Germany