From: Trevor Wiens
On Tue, 15 Mar 2005 07:05:49 +0000 (GMT)
Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
Cross-validation assumes exchangeability of units. You can
your own code (lots of examples in MASS), but first you
prove the validity of what you are attempting. For
chunks in the middle of a time series is not valid unless
somehow takes the temporal structure into account (and glm
Yes, I'm aware of that and I do have a number of predictors
which vary with time (from year to year such as precipitation
or properly timed vegetation indices from each year....) so
that isn't my problem. Also my spatial blocking is also valid
(distinct partitions of the study area). I'm also aware of
the problems of spatial autocorrelation and have taken some
measures to deal with that. I am however rather new at R and
not a statistician, so I am heavily reliant on books such as
Hosmer and Lemeshow or Manley(Resource selection by Animals)
on procedure. Unforunately, they are not S-plus or R oriented
so I have some difficulty translating those ideas to R.
You mention lots of examples in MASS regarding
cross-validation, but I can't find them. Perhaps I'm looking
in the wrong spot. I've done help.search('validation'), ....
and found nothing that seemed obviously applicable to my
problem. I suppose I should pick up a copy of your books
which would probably be very helpful. However, if it isn't
too much trouble. I would really appreciate a bit more direct help.