title: glmer warning: parameters or bounds appear to have different scalings
body: <p>I get the following warning from <code>glmer</code>:</p>
  
  <pre><code>m &lt;- glmer(cbind(Y, N) ~ c1 + c2 + c3 + (1|g1:Year) + (1 + c1 + c2|g1) + (1|g1:Site), 
      family = binomial, data = data, 
      control = glmerControl(optimizer ='optimx', optCtrl=list(method='nlminb'))) 
  
  # Warning in optimx.check(par, optcfg$ufn, optcfg$ugr, optcfg$uhess, lower,  :
  #   Parameters or bounds appear to have different scalings.
  #   This can cause poor performance in optimization. 
  #   It is important for derivative free methods like BOBYQA, UOBYQA, NEWUOA.
  </code></pre>
  
  <p>This is interesting, since all my covariates are scaled (c1: mean = 5.410769e-16, sd = 1), (c2: mean = -2.411114e-16, sd = 1), (c3: mean = 7.602661e-18, sd = 1).</p>
  
  <ol>
  <li><strong>What does this is warning actually mean?</strong> All my covariates are scaled, see above. Scaling them again won't fix it.</li>
  <li><strong>Shall I have to be concerned about this warning in the sense my model could return unreliable estimates?</strong> I got no other warnings or errors.</li>
  </ol>
  
  <p>Thank you!</p>
  
  <p>PS: note - the warning seems to be kinda non-deterministic, on certain data sets in different runs I've observed it sometimes is present and sometimes isn't.</p>