Re: [NMusers] ETAs & SIGMA in external validation

From: Leonid Gibiansky <lgibiansky_at_quantpharm.com>
Date: Fri, 6 Apr 2018 13:51:55 -0400

It would be better to use

$EST METHOD=1 INTERACTION MAXEVAL=0

(at least if the original model was fit with INTERACTION option and
residual error model is not additive).

One option is to use Para = THETA * EXP(ETA)
You would be changing the model, but the model is not too good any way
if you need to restrict Para > 0 artificially.

SIGMA should be taken from the model.

Leonid


On 4/6/2018 12:32 PM, Tingjie Guo wrote:
> Dear NMusers,
>
> I have two questions regarding the statistical model when performing
> external validation. I have a dataset and would like to validate a
> published model with POSTHOC method i.e. $EST METHOD=0 POSTHOC MAXEVAL=0.
>
> 1. The model added etas in proportional way, i.e. Para = THETA * (1+ETA)
> and this made the posthoc estimation fail due to the negative individual
> parameter estimate in some subjects. I constrained it to be positive by
> adding ABS function i.e. Para = THETA * ABS(1+ETA), and the estimation
> can be successfully running. I was wondering if there is better workaround?
>
> 2. OMEGA value influences individual ETAs in POSTHOC estimation. Should
> we assign $SIGMA with model value or lab (where external data was
> determined) assay error value? If we use model value, it's
> understandable that $SIGMA contains unexplained variability and thus it
> is a part of the model. However, I may also understand it as that model
> value contains the unexplained variability for original data (in which
> the model was created) but not for external data. I'm a little confused
> about it. Can someone help me out?
>
> I would appreciate any response! Many thanks in advance!
>
> Your sincerely,
>
> Tingjie Guo
>

Received on Fri Apr 06 2018 - 13:51:55 EDT

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