title: Get data observations used by regression in R (plm)
body: <p>I am estimating a panel model with the package <code>plm</code>. 
  Some of the individuals in the panel do not have data for all the explanatory variables, so they are excluded from the regression. 
  How could I see which particular observations have been used for the estimation?</p>
  
  <p>In Stata the usual command is <code>e(sample)</code>. What is the equivalent in R?</p>