[R] gam function & time trend spline

Roger Peng rdpeng at gmail.com
Tue Jul 3 02:43:27 CEST 2007

If you're looking only at winter days then you probably don't need to
remove seasonal trends, do you?


On 7/2/07, Kevin Sorensen <ksorensen84 at yahoo.com> wrote:
> I've been doing a simple time-series analysis looking
> at the relationship between daily pneumonia
> hospitalizations and daily temperature.  To mimic some
> of the literature, I've been including a time-trend to
> try to account for normal cyclical trends in
> hospitalization.  So I've been using a function that
> looks something like this:
> gam(pneucount ~ temp_f +
> s(day,bs="cr",k=(4*totalyears)+1),
> day being the enumerated day in the analysis (1-365
> for a 1 year analysis).
> This seems to work well enough.  What troubles me is
> when I think about doing an analysis focusing on
> winter days using more than one year of data.  If I
> just delete the summer days from the dataset, the time
> trend spline is trying to anneal counts from the end
> of one winter with the beginning of another, which
> doesn't seem right to me.
> What's the route to a statistically defensible result?
>  Is it as simple as using the subset option?  Or would
> I need to create indicator variables for each winter
> I'm interested and work in a by statement somehow
> (with an extra term for the levels of that indicator,
> I assume)?
> Thanks in advance for helping a Epi student who's
> being exposed to all this for the first time.
> Sincerely,
> Kevin Sorensen
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Roger D. Peng  |  http://www.biostat.jhsph.edu/~rpeng/

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