title: deal with NA's when using tapply
body: <p>Consider the following example:</p>
  
  <pre><code>Factor &lt;- c(rep('Male', 10),rep('Female', 10))
  Age &lt;- sample(30:80,20)
  
  df1 &lt;- data.frame(Factor, Age)
  
  with(df1, tapply(Age, Factor, mean))
  </code></pre>
  
  <p>The final command gives us the average age for both sexes. Now let say one inout is marked with a NA. How can we overcome this issue?</p>
  
  <pre><code>df1$Age[15] &lt;- NA
  with(df1, tapply(Age, Factor, mean))
  </code></pre>