title: Estimate Cohen's d for effect size
body: <p>given two vectors:</p>
  
  <pre><code>x &lt;- rnorm(10, 10, 1)
  y &lt;- rnorm(10, 5, 5)
  </code></pre>
  
  <p>How to calculate Cohen's d for effect size?</p>
  
  <p>For example, I want to use the <a href="http://www.statmethods.net/stats/power.html">pwr package</a> to estimate the power of a t-test with unequal variances and it requires Cohen's d.</p>