Nick,
Unfortunately I am away from a computer with Stata installed and have been tapping this out on a cell phone!
if dose=0 then the logistic equation reduces to
b[zdose] + b[cons]
If dose=1, it reduces to just
b[cons].
Agreed. Units are irrelevant.
I'll tell you what: When I get home on 11/1, i'll start a new thread with a more convincing example (unless somebody wants to agree with me before then)...
Dan
-----Original Message-----
From: "Nick Cox" <[email protected]>
Subj: RE: st: reverse prediction - confidence interval for x at given y in nonlinear model
Date: Fri Oct 26, 2007 11:23 am
Size: 2K
To: <[email protected]>
We have been at cross purposes.
Your log(0) as 0 is, I see, only a trick that keeps
the zeros in the frame _so long as you also have a dummy
for 0_.
My point is that it does not help in problems in which there is no dummy
for
0.
Suppose your doses are 0 0.1 0.2 0.3 0.4 0.5
in whatever units are being used. Neither Stata nor the mathematics
knows about the units.
Then the recipe cond(dose == 0, 0, log(dose)) is not even monotonic
in dose. 0 is treated as if were larger than any other positive
value in the data!
Also if any dose is 1, this recipe treats 0 and 1 as
equivalent.
Nick
[email protected]
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