Programming the statistical procedures from SAS

Can I use a covariate in NLMIXED

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Contributor
Posts: 45

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

Frequent Contributor
Posts: 140

Re: Can I use a covariate in NLMIXED

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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