title: The confidence interval by the intercept with linear regression in R
body: <p>Let's say that i have two variables <strong>weight</strong> and <strong>age</strong>, i have to find the confidence interval with level 99% by this case:</p>
  
  <ol>
  <li>By the ordinate (Y-Axis), if we did a linear regression <code>a=lm(weight~age)</code></li>
  </ol>
  
  <p>I know that the ordinate is directly the <strong>intercept</strong> but why this won't work:</p>
  
  <pre><code>predict(a, newdata=data.frame(age=intercept), interval='confidence',
  level=0.99)
  </code></pre>
  
  <p>Why this is incorrect? I would like to know the correct commands for these cases. </p>