title: R: cannot predict specific value
body: <pre><code>&gt; age &lt;- c(23,19,25,10,9,12,11,8)
  &gt; steroid &lt;- c(27.1,22.1,21.9,10.7,7.4,18.8,14.7,5.7)
  &gt; sample &lt;- data.frame(age,steroid)
  &gt; fit2 &lt;- lm(sample$steroid~poly(sample$age,2,raw=TRUE))
  &gt; fit2
  
  Call:
  lm(formula = sample$steroid ~ poly(sample$age, 2, raw = TRUE))
  
  Coefficients:
  (Intercept)                        -27.7225     
   poly(sample$age, 2, raw = TRUE)1    5.1819  
   poly(sample$age, 2, raw = TRUE)2   -0.1265  
  
  
  &gt; (newdata=data.frame(age=15))
   age
  1  15
  
  &gt; predict(fit2,newdata,interval="predict")
      fit       lwr      upr
  1 24.558395 17.841337 31.27545
  2 25.077825 17.945550 32.21010
  3 22.781034 15.235782 30.32628
  4 11.449490  5.130638 17.76834
  5  8.670526  2.152853 15.18820
  6 16.248596  9.708411 22.78878
  7 13.975514  7.616779 20.33425
  8  5.638620 -1.398279 12.67552
  Warning message:
  'newdata' had 1 rows but variable(s) found have 8 rows 
  </code></pre>
  
  <p>Why does the <code>predict</code> function unable to predict for age=15?</p>