Re: Averaging probabilities?

On Mar 19, 2:57 pm, rocketD <dar...@xxxxxxxxx> wrote:
Hi all.

<b>Background:</b> I'm working on a study looking at neonatal
hypoglycemia as a binary outcome, using infants who have neonatal
hypoglycemia (cases) matched for comparison with infants who don't
(controls). I've run a univariate model using conditional logistic
regression in Stata 9, and for each observation, have predicted the
conditional probability of developing neonatal hypoglycemia given
gestational age. The probability value output by Stata is called

P(Neonatal Hypoglycemia | Single outcome w/in group)

and I want to plot this against gestational age to show that there is
a steep negative slope up to a certain gestational age, after which
point it levels out. (Translation: risk of hypoglycemia decreases as
gestational age increases, to a point, then becomes stable.) I'd like
to do this cleanly, with one summary probability, like an average,
[that is, one y-value] for each gestational age (x-value), rather than
the 30 or so I have for each category now.

--- snip ---

Does this mean that you treated age as a continuous variable in your
model, but now you want to use age categories in the plot? If age was
treated as a continuous variable, why not treat it that way in your
plot too. I.e., make a scatter-plot with X = age (selected values),
and Y = the fitted probability (or log-odds) of the outcome? If your
model treated age as continuous (and did not contain age-squared or
any other higher order terms), then such a plot with Y = log-odds of
the outcome should give you a straight line. With Y = the fitted
probability of the outcome, it will be curvilinear, as you said.

--
Bruce Weaver
bweaver@xxxxxxxxxxxx
www.angelfire.com/wv/bwhomedir
"When all else fails, RTFM."
.

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