title: Is it appropriate to calculate r-squared of robust regression using rlm
body: <p>I am using rlm function from MASS to perform a robust regression. Unlike lm,the summary function does not return a valure for r-squared.</p>
  
  <p>Is therefore appropriate to calculate this using <code>1 - sum(residual^2)/(sum((Y-mean(Y))^2)?</code></p>
  
  <p>(apologies for the equation, I could not figure how to write this in a better formatted way)</p>