title: What is the meaning of the "Residual standard error" in summary() for lm() regression in R?
body: <p>When I use R to run lm() regression, I get the "Residual standard error" from summary(). Why there is just one value of the residual standard error rather than the list of residual standard errors for each observation? </p>
  
  <p>What is the meaning of this value showed in summary()? Is the "Residual standard error" showed in summary() the mean of the list of residual standard errors for each observation? Thanks. </p>
  
  <pre><code>Residual standard error: 0.8498 on 44848 degrees of freedom
    (7940 observations deleted due to missingness)
  Multiple R-squared:  0.4377,    Adjusted R-squared:  0.4375
  </code></pre>