Re: [NMusers] Combined residual model and IWRES.

Paul,
In thetas, SE% refers to SE% of SD while in OMEGAs it refers to
variances (SD^2). To make them identical, use

IWRES=(DV-IPRED)/SQRT(F*F*THETA(3)+THETA(4))
Y=F*(1+SQRT(THETA(3))*ERR(1))+SQRT(THETA(4))*ERR(2).

also, if you take the square of SD SE%, you will see that it nearly
matches SE% for OMEGAs, as it should
Leonid

--------------------------------------
Leonid Gibiansky, Ph.D.
President, QuantPharm LLC
web: www.quantpharm.com
e-mail: LGibiansky at quantpharm.com
tel: (301) 767 5566




Paul Westwood wrote:
> Dear All,
>
> I'm working on a one-compartment iv model. I have used a model for the
> IWRES from a discussion from May 2001 (thanks to Mats Karlsson, Niclas
> Jonsson and Nick Holford), where you use thetas to obtain the sigmas
> when using a combined residual model:
>
> IPRED=F
> IRES=DV-IPRED
> IWRES=(DV-IPRED)/SQRT(F*F*THETA(3)*THETA(3)+THETA(4)*THETA(4))
> Y=F*(1+THETA(3)*ERR(1))+THETA(4)*ERR(2).
>
> where you then fix the sigmas to 1. I obtained the following results for
> a particular base model:
>
> ETA = 3.16 28.1 0.0804 0.163
> ETASD = 1.23693 1.42478
> ERRSD = 1 1
> THETA:se% = 23.9 32.0 39.9 23.3
> OMEGA:se% = 18.4 33.8
> SIGMA:se% = 0.0 0.0
>
> I then ran the same model but using the normal code in the $ERROR
> section to see if there was any difference in the final estimates:
>
> IPRED=F
> IRES=DV-IPRED
> Y=F*(1+ERR(1))+ERR(2)
>
> and obtained these results:
>
> THETA = 3.16 28.1
> ETASD = 1.23693 1.42478
> ERRSD = 0.0803741 0.163095
> THETA:se% = 23.5 32.0
> OMEGA:se% = 18.5 33.9
> SIGMA:se% = 79.3 45.9
>
> Here are few questions: 1.Can anyone tell me why the standard errors for
> the thetas in model 1 and the standard errors for the sigmas in model 2
> differ so significantly? 2.Why does the algorithm used to obtain the
> standard errors for the sigmas differ so much from that used to obtain
> standard errors for the thetas, and how? 3.What are the implications
> when then using INTERACTION? 4....and finally, which model should i use?
>
> Thankyou in advance for any light that can be shed.
>
> Best,
>
> Paul Westwood.
>
>
Received on Thu Feb 14 2008 - 14:53:16 EST