title: Getting Warning: " 'newdata' had 1 row but variables found have 32 rows" on predict.lm
body: <p>I found peculiarity while using predict and lm function in R. I got different results for data frame and vector for same data. </p>
  
  <p><strong><em>DataFrame code:</em></strong></p>
  
  <pre><code>data(mtcars)
  fitCar&lt;-lm(mtcars$mpg~mtcars$wt)
  predict(fitCar,
          data.frame(x=mean(mtcars$wt)),
          interval="confidence")
  </code></pre>
  
  <p><strong>Output:</strong></p>
  
  <pre><code>     fit       lwr      upr
  1  23.282611 21.988668 24.57655
  2  21.919770 20.752751 23.08679
  3  24.885952 23.383008 26.38890
  4  20.102650 19.003004 21.20230
  5  18.900144 17.771469 20.02882
  6  18.793255 17.659216 19.92729
  7  18.205363 17.034274 19.37645
  8  20.236262 19.136179 21.33635
  9  20.450041 19.347720 21.55236
  10 18.900144 17.771469 20.02882
  11 18.900144 17.771469 20.02882
  12 15.533127 14.064349 17.00190
  13 17.350247 16.104455 18.59604
  14 17.083024 15.809403 18.35664
  15  9.226650  6.658271 11.79503
  16  8.296712  5.547468 11.04596
  17  8.718926  6.052112 11.38574
  18 25.527289 23.927797 27.12678
  19 28.653805 26.519252 30.78836
  20 27.478021 25.554415 29.40163
  21 24.111004 22.715653 25.50635
  22 18.472586 17.319886 19.62529
  23 18.926866 17.799465 20.05427
  24 16.762355 15.452833 18.07188
  25 16.735633 15.423002 18.04826
  26 26.943574 25.112491 28.77466
  27 25.847957 24.198041 27.49787
  28 29.198941 26.963760 31.43412
  29 20.343151 19.242185 21.44412
  30 22.480940 21.268498 23.69338
  31 18.205363 17.034274 19.37645
  32 22.427495 21.219818 23.63517
  </code></pre>
  
  <p>Warning message:</p>
  
  <blockquote>
    <p>'newdata' had 1 row but variables found have 32 rows </p>
  </blockquote>
  
  <p>When I separate out both data into vector, i got different answer</p>
  
  <p><strong><em>Code for vector</em></strong> </p>
  
  <pre><code>predict(fit,data.frame(x=mean(x)), interval="confidence")
  </code></pre>
  
  <p><strong>Output:</strong></p>
  
  <pre><code>    fit   lwr   upr
  1 20.09 18.99 21.19
  </code></pre>
  
  <p>What is the reason for this difference?</p>