# Re: [R] round(1.5) = round(2.5) = 2?

From: Toby Marthews <Toby.Marthews_at_lsce.ipsl.fr>
Date: Sun, 15 Jun 2008 11:58:29 +0200 (CEST)

Hi Markus,

The R function round() uses the round-to-even method, which is explained on http://en.wikipedia.org/wiki/Rounding#Round-to-even_method.

If you would like instead "traditional rounding" then you should add 0.5 and take the integer part, as is suggested in the examples on ?round, e.g.

The reason for the IEEE standard is to do with signal processing and the bias introduced by traditional rounding if you have a lot of data points whose decimal expansion ends in ....5.

Personally, I am a mathematician and satistician and I find that in 99% of cases traditional rounding is what is required (basically always except some very specific examples involving very large sets of data) so for R to have put the IEEE standard as the default (rather than, say, an option) is a bit odd. However, R's benefits and advantages by far outweigh its little oddities, as I presume you know since you are using it.

Effectively, I never use the round() command and always calculate using the floor function.

Toby Marthews

Le Dim 15 juin 2008 11:26, Markus Didion a écrit :
> Dear R-users
>
> with a bit of grief I had to repeat an extensive analysis because I
> did not suspect (and therefore did not read the documentation) that
> round was implemented as "for rounding off a 5, the IEC 60559 standard
> is expected to be used, 'go to the even digit'", resulting in
> round(1.5) = 2
> round (2.5) = 2.
>
> As a non-mathematician I am both puzzled and intrigued by this rule as
> it is against what I have learned in my math courses, i.e.
> round(1.5) = 2
> round (2.5) = 3.
>
> I would like to understand the reason behind this rule.
>
>
> Markus
>
> --
>
> Markus Didion
>
> Wald?kologie Forest Ecology
> Inst. f. Terrestrische Oekosysteme Inst. of Terrestrial Ecosystems
> Departement Umweltwissenschaften Dept. of Environmental Sciences
> Eidg. Technische Hochschule Swiss Fed. Inst. of Technology
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>
> Tel +41 (0)44 632 5629 Fax +41 (0)44 632 1358
> Email markus.didion_at_env.ethz.ch
> homepage: http://www.fe.ethz.ch/people/didionm

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