title: Calculate skew and kurtosis by year in R
body: <p>I have a table that looks like this:</p>
  
  <pre><code>start_table &lt;- data.frame("Water_Year" =  c("1903", "1903", "1904", "1904"), "X" = c(13, 11, 12, 
  15), "Day" = c(1, 2, 1, 2))
  </code></pre>
  
  <p>(The 'Day' column is not involved in my skew &amp; kurtosis calculation, it is just in my table)</p>
  
  <p>I would like a table that calculates the skew and kurtosis values grouped by year:</p>
  
  <pre><code>end_table &lt;- data.frame("Water_Year" =  c("1903", "1904"), "Skew" = c("skew_number_here", 
  "skew_number_here"), "Kurtosis" = c("kurtosis_number_here", "kurtosis_number_here"))
  </code></pre>
  
  <p>I can't figure out how to group it by year to perform these calculations.</p>