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RE: st: Collapsing data to daily data


From   Nick Cox <[email protected]>
To   "'[email protected]'" <[email protected]>
Subject   RE: st: Collapsing data to daily data
Date   Wed, 2 Mar 2011 19:27:17 +0000

Interesting. 

Not pertinent really, but still intriguing: 38 cm (well, 381 mm) looks to me like 15 inches in disguise. Whether deer use metric or non-metric units in reacting to snow is no doubt a subject for future research. Perhaps this means that there is some device that takes up to 15 inches' worth of snow, after which it is deemed to overflow. 

Nick 
[email protected] 

Brigham Whitman

Yes, at first I wasn't going to collapse my data for a survival
analysis.  It is a huge data set (68 individual white-tailed deer with
a total of 72,000 data points with up to 4 data points per day per
deer), but Stata and R could handle it fine.  But my advisors for my
thesis said I should collapse the data, and another man I contacted
who has done the same analysis suggested I should collapse it, so I
did.  That man had mentioned something about how I might inadvertently
be increasing the amount of time each animal is at risk if I use
multiple locations per animal per day.  I can't say that I totally
understand why that is.  It does make more sense to me now to look at
daily data, and I've incorporated a 'Cumulative days of snow depth
over 38 cm' variable and running averages for certain variables over
14 and 28 day moving windows, which I am not sure how I would do if I
hadn't collapsed the data.  But yes, I should have a better idea of
why I would "coarsen" my data and lose some of that accuracy.  Thank
you for the input.


On Tue, Mar 1, 2011 at 9:44 AM, Nick Cox <[email protected]> wrote:

> I don't see that you need to coarsen your data for a survival analysis -- unless the sheer size of the dataset is problematic.
>
> It could even be that time of day data has a secondary bearing on survival...
>
> Nick
> [email protected]
>
> Brigham Whitman
>
> Yes, these options worked out for me, thank you.
>
> I am collapsing the data set to prepare it for a Cox proportional
> hazard model.  Each data point already has a few variables (distance
> to cover, distance traveled, snow depth encountered) with measurements
> for each point that I can average together for each day before I make
> the model.

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