On Sun, 12 Oct 2008, David Winsemius wrote:
Two follow-up questions:
A) I get an error message when using Harrell's describe() function on one of
my variable, telling me that sum() is not meaningful for a difftime object.
Why should sum() not be meaningful for a collection of interval lengths?
That's not what it actually says. It says it is 'not defined' -- it could
be defined but it has not been. Just add a function sum.difftime() with
appropriate code (and watch out that different difftime objects can be in
different units).
describe(pref900)
Error in Summary.difftime(c(1075, 3429, 2915, 2002, 967, 1759, 532, 589, :
'sum' not defined for "difftime" objects
summary() is informative and throws no error, but does not report means.
Even with na.rm=TRUE, sum fails:
sum(pref900$deatht, na.rm=TRUE)
Error in Summary.difftime(c(NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, NA, :
'sum' not defined for "difftime" objects
My interest in the sum of difftime objects comes from my interest in
calculating the number of person-years of observation in various categories.
I have durations created by subtracting times.
B) The help pages are not particularly expansive regarding the output of
deltat() but your answer suggests that it should work on non-time objects as
well? Am I correct in assuming you meant that diff(x)/deltat(x) should be
meaningful for any numeric x.
--
David Winsemius
R 2.7.1 / Mac OS 10.5.4 / Intel CPUs
s>
On Oct 12, 2008, at 10:34 AM, Gabor Grothendieck wrote:
?deltat
On Sun, Oct 12, 2008 at 9:45 AM, Oliver Bandel
<[EMAIL PROTECTED]> wrote:
Zitat von Gabor Grothendieck <[EMAIL PROTECTED]>:
If you simply want successive differences use diff:
x <- seq(4)^2
diff(x)
tx <- ts(x)
diff(tx)
[...]
Oh, cool, thanks.
But what about diff / delta_t ?
Do I have to calculate it by my own, or is there
already a function for making a difference-qoutient?
This would be fine to have, because for example
coming from space vs. time to velocity vs. time
and acceleration vs. time (and further derivatives)
are also a time-series.
The possibility of using the advantages of the time series class here,
would be fine.
Ciao,
Oliver
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--
Brian D. Ripley, [EMAIL PROTECTED]
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 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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