Hi Nick,
I am not sure how you build the models but I am using convergence,
relative standard errors, correlation matrix of parameter estimates
(reported by the covariance step), and correlation of random effects
quite extensively when I decide whether I need extra compartments, extra
random effects, nonlinearity in the model, etc. For me they are very
useful as diagnostic of over-parameterization. This is the direct
evidence (proof?) that they are useful :)
For new modelers who are just starting to learn how to do it, or have
limited experience, or have problems on the way, I would advise to pay
careful attention to these issues since they often help me to detect
problems. You seem to disagree with me; that is fine, I am not trying to
impose on you or anybody else my way of doing the analysis. This is just
an advise: you (and others) are free to use it or ignore it :)
Thanks
Leonid
--------------------------------------
Leonid Gibiansky, Ph.D.
President, QuantPharm LLC
web: www.quantpharm.com
e-mail: LGibiansky at quantpharm.com
tel: (301) 767 5566
Nick Holford wrote:
Mats,
[This thread contains several quite different directions so I've decided
to try and split them]
I'm happy to agree to disagree but in fact I think we agree on the first
issue.
"What has been shown by many of us is that with bootstraps or simulation
under the same model and same design, convergence is not a reliable tool
for detecting quality of parameter estimates. "
The second issue:
"That is far from showing its lack of value to detect overestimation in
other types of situations, most importantly model building."
i.e. convergence/covariance is of value in model building lacks any
evidence that I am aware of. I'm not sure what you mean by
overestimation but I am guessing it something like the term
overparameterization that Leonid used.
I'd like to hear from you and Leonid how exactly you define these terms
"overestimation" and "overparameterization". Can you provide a test that
says a model is being "overestimated" or the model is "overparameterized"?
Nick
Mats Karlsson wrote:
Hi Nick,
Maybe Leonid's suggestion to agree to disagree was a good one but here
we go
again :)
See below
Mats
Mats Karlsson, PhD
Professor of Pharmacometrics
Dept of Pharmaceutical Biosciences
Uppsala University
Box 591
751 24 Uppsala Sweden
phone: +46 18 4714105
fax: +46 18 471 4003
