I have what seems to be a straightforward question involving a conditional
probability, but I must be missing something because I can't quite get a
handle on it. Let's say I have treatment and control groups with
individuals preassigned to each, with T individuals in the treatment group
and C in the control group. I observe mortality after some period of time,
with t of T dying in the treatment group and c of C in the control group.
I would like a measure of the probability of death due to the treatment,
over and above (in some sense) the probability of death in the control group.
I know that P(x of T) is hypergeometric, assuming that the probabilities of
death for treatment and control are identical, so I know how to determine
whether (t of T) is significantly greater than (c of C). And I've just
verified that this probability is the same as the chi-square probability
for the 2 x 2 contingency table. But how do I measure this effect? As a
simple difference between the probabilities for the two groups?
I initially guessed that the value I wanted was just P(death | treatment),
but of course this turns out to be just the ratio t/T, which contains no
information about the control group. I'm sure this must be commonly done,
as, for example, in estimating the additional probability of death at a
particular age due to smoking, but I've scanned the resources (texts,
personnel, etc.) I have available and can't find the relevant information.
Can someone point me in the right direction?
Thanks in advance.
Rich Strauss
========================
Dr Richard E Strauss
Biological Sciences
Texas Tech University
Lubbock TX 79409-3131
Email: [EMAIL PROTECTED] (formerly [EMAIL PROTECTED])
Phone: 806-742-2719
Fax: 806-742-2963
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