# RE: Mixture model with logistic regression

From: Bob Leary <Bob.Leary>
Date: Sat, 20 Feb 2016 13:45:08 +0000

This sounds like a good case for a nonparametric method - if you use the o=
ne in NONMEM, you might try
expanding Omega to counter shrinkage. The versions in USC*PACK and PHOENI=
X NLME optimize over
both support point positions and probabilities, so this is not necessary wi=
th those methods.
________________________________
From: owner-nmusers
of Mark Sale [msale
Sent: Friday, February 19, 2016 4:30 PM
To: nmusers
Subject: [NMusers] Mixture model with logistic regression

Has anyone every tried to use a mixture model with logistic regression? I h=
ave data on a AE in several hundred patients, measured multiple times (10-2=
0 times per patient). Examining the data it is clear that, independent of =
drug concentration, there is very wide distribution of this AE, 68% of the =
patients never have the AE, 25% have it about 20% of the time and the rest =
have it pretty much continuously, regardless of drug concentration. (in or=
dinary logistic regression, just glm in R, there is also a nice concentrati=
on effect on the AE in addition). Running the usual logistic model, not s=
urprisingly, I get a really big ETA on the intercept, with 68% of the peopl=
e having ETA small negative, 25% ETA ~ 1 and 7% ETA ~ 10. No covariates see=
m particularly predictive of the post hoc ETA. I thought I could use a mix=
ture model, with 3 modes, but it refused to do that, giving me essentially =
0% in the 2nd and 3rd distribution, still with the really large OMEGA for t=
he intercept. Even when I FIX the OMEGA to a reasonable number, I still ge=
t essentially no one in the 2nd and 3rd distribution. I tried fixing the f=
raction in the 2nd and 3rd distribution (and OMEGA), and it still gave me a=
very small difference in the intercept for the 2nd and 3rd populations.

Is there an issue with using mixture models with logistic regression? I'm j=
ust using FOCE, Laplacian, without interaction, and LIKE.

Any ideas?

Mark

Mark Sale M.D.
Vice President, Modeling and Simulation
Nuventra, Inc. ™
2525 Meridian Parkway, Suite 280
Research Triangle Park, NC 27713
Office (919)-973-0383
msale
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Received on Sat Feb 20 2016 - 08:45:08 EST

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