On Oct 17, 2011, at 8:04 AM, Sarah Goslee wrote:
Hi,
On Sun, Oct 16, 2011 at 6:24 PM, Bob Briggs <rbrig...@hotmail.com>
wrote:
Hello
I'm wanting to understand more about ptrend (a statistical
explanation via an internet website if possible) and also to know
what package in R would produce a ptrend.
Appreciate any help I can get on this as I'm trying to read and
understand an epidemiological paper and data and reproduce similar
results for my own understanding.
Given that I'm not an epidemiologist, and have no idea what a ptrend
is, and that you didn't actually explain what kind of beast it might
be:
It seems likely that it is a test for linear trend in odds ratios or
rate ratios in an ordered set of proportions. It was taught us
(epidemiologists) in the courses before we got our hands on logistic
regression. In my training it was called the Cochran-Armitage test for
trend:
http://en.wikipedia.org/wiki/Cochran-Armitage_test_for_trend
You get such test when you use prop.trend.test {stats}.
It is basically a linear regression (or a Pearson correlation) with
a "chi-square test" using the weighted departures of counts in cells
from an expected value set by the overall proportion. (Look at the
code for prop.trend.test. It makes perfect sense that a set of
squared departures from an expected value set by the mean could be
cast as a "chi-square test".) I do not know of any advantages for the
test over logistic regression or Poisson regression. You can take an
ordered factor (or a non-ordered on if the levels are properly set up,
coerce to numeric and do logistic regression with the numeric result
and get pretty much the same result, and you would be doing so in the
context of a much more flexible modeling environment. So I see it
mainly as of historical interest, something to use when you only have
a device that cannot run R.
--
David.
If you go to http://www.rseek.org and search for ptrend, you will get
several R packages that use that term, a few posts from the R-help
list, and a pile of websites that might be helpful.
Sarah
Thanks
Meredith
David Winsemius, MD
West Hartford, CT
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