title: Stata: Choose which interaction to omit using factor variables and fixed effects
body: <p>Using factor variables in Stata in a regression it's easy to choose which main effect to omit. However, as in the below example, sometimes I wish to choose which additional collinear factor variable to omit, where here the collinearity is due to the fixed effects. I thought the ib4 notation would do it, but Stata seems to ignore this, instead omitting the interaction between indicators for treat=1 and t=8. How can I force omission of the interaction between indicators for treat=1 and t=4? </p>
  
  <pre><code>clear
  set obs 100
  gen id=_n
  gen treat=runiform()&gt;.5
  expand 8
  bysort id: gen t=_n
  gen y_0=rnormal()+3*t
  gen y_1=rnormal()+5*t+5
  gen post=t&gt;4
  gen posttreat=post*treat
  gen y=cond(posttreat==1,y_1,y_0)
  areg y ib4.t i.treat#ib4.t, absorb(id)
  </code></pre>