From p.dalgaard at biostat.ku.dk  Sat Sep 15 17:56:32 2007
From: p.dalgaard at biostat.ku.dk (Peter Dalgaard)
Date: Sat, 15 Sep 2007 17:56:32 +0200
Subject: [Rd] Date vs date
In-Reply-To: <Pine.LNX.4.64.0709150756500.8538@gannet.stats.ox.ac.uk>
References: <200709141722.l8EHMHF02634@hsrnfs-101.mayo.edu>	<46EAF699.1010000@biostat.ku.dk>	<f8e6ff050709141419g5637437bkc52dd809120e7760@mail.gmail.com>
	<Pine.LNX.4.64.0709150756500.8538@gannet.stats.ox.ac.uk>
Message-ID: <46EC00B0.2060105@biostat.ku.dk>

Prof Brian Ripley wrote:
>>> Arguably, it should (Is this a difftime object? Which units?).
>>> I'd advise against numeric operation on difftime objects in general,
>>> because of the unspecified units. These are always "days" when working
>>> with Date objects, but with general time objects it is not predictable.
>>> So I'd recommend sqrt(as.numeric(temp, units="days")).
>>>       
>
> It fails by design.  Using sqrt() on a measurement that has an arbitrary 
> origin would not have been good design.
Er, _diff_time objects do have a well-defined origin at zero. Taking 
square roots (or exp, or...) of a number which  could be weeks as well 
as days is the real issue, I think.

-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907



[Rd] Date vs date
Peter Dalgaard