Dear All,
   
  I don't know whether this question is appropriate for this list or not. My question is as follows:
   
  I am fitting a non-linear mixed model  using the SSlogis  function.  When the parameters  xmid and scal are both considered as random (let us say model M1), the p-value for my covariate of interest is greater than 0.05 (non-significant). However, when only the xmid is considered as random (Model M2), the p-value is less than 0.05 and my covariate becomes highly significant. The introduction of random effects of course increases the variability in the fixed effects but does not affect their Bias as I understand. My question is: Is it then legitimate to use the model with only xmid as random effects (model M2 since it gives me significant results)? knowing that the AIC for M1 is lower than that one for M2 meaning that M1 is slightly fitting better  the data than M2
   
  Many thanks in advance
   
  Bernard,
   
    

       
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