# Re: Reposted: st: cytel challenge

 From Jose Maria To statalist@hsphsun2.harvard.edu Subject Re: Reposted: st: cytel challenge Date Mon, 29 Mar 2004 21:44:14 -0300

```Tero T Kivela escreveu:

```
On Wed, 21 Nov 2001, William Gould wrote:

Lee Sieswerda <Lee.Sieswerda@tbdhu.com> wrote,

Cytel makes LogXact for doing "exact" logistic regression (www.cytel.com).
In their ads they have something called the "Cytel Challenge"
I take issue with Lee's comment that "Stata fails to produce a converged
model" -- Lee did something wrong -- but I do not take issue with Cytel's
ad (although I have not seen it).

==============================================================================
. blogit diarrhea totno cep cli sex age los, or
note: cephalex~=0 predicts success perfectly
cephalex dropped and 2 obs not used

Logit estimates Number of obs = 2488
LR chi2(4) = 91.48
Prob > chi2 = 0.0000
Log likelihood = -218.30047 Pseudo R2 = 0.1732

------------------------------------------------------------------------------
_outcome | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
clindomy | 9.198602 2.89523 7.05 0.000 4.963739 17.04648
sex | .8263463 .2336678 -0.67 0.500 .474751 1.438329
age | 2.440564 1.176263 1.85 0.064 .948947 6.276803
los | 11.84492 7.113316 4.12 0.000 3.65051 38.43354
------------------------------------------------------------------------------
==============================================================================

What Cytel wants you to notice is Stata's message

note: cephalex~=0 predicts success perfectly
cephalex dropped and 2 obs not used

That is the point of their challange: When cephalex is nonzero, there is
always a positive outcome:

. list if ceph==1

diarrhea totno cephalex clindomy sex age los
17. 1 1 1 0 0 1 1
18. 4 4 1 0 1 1 1

There are a total of 5 patients who were observed with cephalex==1 and all
five patients suffered from diarrhea. How do you interpret that? Does that
mean cephalex==1 always results in diarrhea? Well, of course it does not.
With only five such patients, Cytel's computationally intensive methods were
able to put a confidence interval around the result: [27.52, infinity]. Very
nice. (I would like somebody to explain to me why the point estimate is a
finite 207.40 rather than infinite, but I'm sure Cytel has carefully

In any case, Stata smartly recognized its limitations and estimated the model
conditional on cephalex==0. Some other packages might not have recognized the
problem and gotten messed up in the numerics.

I leave it for you to decide how important it is to put a confidence interval
around cephalexin in this particular case, but without question there are
problems for which doing this kind of thing is important.

-- Bill
wgould@stata.com
*

```Beautiful. Better: Billtiful.
Jose Maria

--
Jose Maria Pacheco de Souza, Professor Titular
Departamento de Epidemiologia
Av. Dr. Arnaldo, 715	cep 01246-904
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www.fsp.usp.br/~jmpsouza
jmpsouza@usp.br

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