Programming the statistical procedures from SAS

Can I use a covariate in NLMIXED

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Can I use a covariate in NLMIXED

Hi,

To add a covariate to NLMIXED, is this as simple as to add to a GLIMMIX model for example?

Thanks,

Marcio

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Posts: 140

Re: Can I use a covariate in NLMIXED

Posted in reply to marcioadg

Nothing in NLMIXED is as simple as it is in GLIMMIX.  However, the NLMIXED documentation provides examples of models with covariates. See e.g. SAS/STAT(R) 9.2 User's Guide, Second Edition  where they give this code for a model with several covariates:

proc nlmixed data=inhaler corr ecorr;

  parms b0=0 b1=0 b2=0 b3=0 sd=1 i1=1 i2=1;

  bounds i1 > 0, i2 > 0;

  eta = b0 + b1*group + b2*time + b3*gt + u;

  if (clarity=1) then p = probnorm(-eta);

  else if (clarity=2) then

  p = probnorm(i1-eta) - probnorm(-eta);

  else if (clarity=3) then

  p = probnorm(i1+i2-eta) - probnorm(i1-eta);

  else p = 1 - probnorm(i1+i2-eta);

  if (p > 1e-8) then ll = log(p);

  else ll = -1e100;

  model clarity ~ general(ll);

  random u ~ normal(0,sd*sd) subject=sub;

  replicate freq;

  estimate 'thresh2' i1;

  estimate 'thresh3' i1 + i2;

  estimate 'icc' sd*sd/(1+sd*sd);

  run;

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