# Re: Piece-wise PD model

From: Ekaterina Gibiansky <EGibiansky>
Date: Wed, 23 Jun 2010 11:20:34 -0400
Brendan,

The implementation of your piece-wise model is not correct. It should be

EFFECT=AEFF*SLOPE1
IF(
AEFF.GT.CHANGE) EFFECT=CHANGE*SLOPE1+(AEFF - CHANGE)*SLOPE2

Regards,
Katya

```Ekaterina Gibiansky, Ph.D.
CEO&CSO, QuantPharm LLC
Web:     www.quantpharm.com
Email:   EGibiansky at quantpharm.com
Tel:     (301)-717-7032 ```

On 6/23/2010 10:20 AM, Brendan Johnson wrote:

NMusers,

In a somewhat similar theme to Hauke’s post, I am having an issue with conditional assignment statements in \$ERROR.  I am trying to fit a piece-wise PD model for a KPD system. A linear PD model is ok, but saturating models, while I expect these to provide a better fit, converge with parameters that essentially replicate a linear model (tried Emax, Power, Exponential). My last ditch attempt was to try a piece-wise model, with 2 linear slopes, estimating the change point.

Something like this

\$PK

CHANGE=THETA(.)

SLOPE1=THETA(.)

SLOPE2=THETA(.)

\$ERROR

A(.)=AEFF                               ;amount in effect compartment

IF(AEFF.LT.CHANGE) THEN

SLOPE=SLOPE1

ELSE

SLOPE=SLOPE2

ENDIF

EFFECT=AEFF*SLOPE

I always find minimization is terminated early, and the gradient for CHANGE is zero at first iteration.  The gradient does have a value at subsequent iterations, but the final estimate of CHANGE (at termination anyway) is usually not far off the initial estimate…I suspect it is not actually being estimated, just floating a bit.  Even if I fix CHANGE to a reasonable value, I see minimization is terminated early.

Is there some trick I am missing here? or is it not possible to estimate a parameter within a conditional assignment statement in \$ERROR?

(seems like you can do this in \$PK when a covariate or time is used in the IF statement)

Thanks for any help,

Brendan Johnson

GlaxoSmithKline, RTP

```

No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 9.0.829 / Virus Database: 271.1.1/2956 - Release Date: 06/22/10 14:36:00

```

No virus found in this outgoing message.
Checked by AVG - www.avg.com
Version: 9.0.829 / Virus Database: 271.1.1/2956 - Release Date: 06/22/10 14:=
36:00

Received on Wed Jun 23 2010 - 11:20:34 EDT

The NONMEM Users Network is maintained by ICON plc. Requests to subscribe to the network should be sent to: nmusers-request@iconplc.com.

Once subscribed, you may contribute to the discussion by emailing: nmusers@globomaxnm.com.