title: Error message: Error in fn(x, ...) : Downdated VtV is not positive definite
body: <p>I'm trying to use the lmer function to create a minimum adequate model. My model is Mated ~ Size * Attempts * Status + (random factor).</p>
  <pre><code>as.logical(Mated)
  as.numeric(Size)
  as.factor(Attempts)
  as.factor(Status)
  </code></pre>
  <p>(These have all worked on previous models)</p>
  <p>So after all that I try running my model:</p>
  <pre><code>Model1&lt;-lmer(Mated ~ Size*Status*Attempts + (1|FemaleID),data=mydata)
  </code></pre>
  <p>And it can be submitted without fault.It's only when I try to apply this update that it goes wrong:</p>
  <pre><code>Model2&lt;-update(Model1, REML=FALSE)
  </code></pre>
  <p>Here is the error message supplied:</p>
  <blockquote>
  <p>Error in fn(x, ...) : Downdated VtV is not positive definite</p>
  </blockquote>
  <p>If I make a third model without the interaction and do an ANOVA between that and model one, then it says the two are significantly different.</p>
  <pre><code>Model3&lt;-update(Model1,~.-Size:Status:Attempts
  anova(Model1,Model3)
  </code></pre>
  <p>What am I doing wrong? Is the three way interaction really significant or have I made some mistake?</p>
  <p>Thank you</p>