title: Calculate poisson probability percentage
body: <p>When you use the POISSON function in Excel (or in OpenOffice Calc), it takes two arguments:</p>
  
  <ul>
  <li>an integer</li>
  <li>an 'average' number</li>
  </ul>
  
  <p>and returns a float.</p>
  
  <p>In Python (I tried RandomArray and NumPy) it returns an array of random poisson numbers.
  What I really want is the percentage that this event will occur (it is a constant number and the array has every time different numbers - so is it an average?).</p>
  
  <p>for example:</p>
  
  <pre><code>print poisson(2.6,6)
  </code></pre>
  
  <p>returns <code>[1 3 3 0 1 3]</code> (and every time I run it, it's different).</p>
  
  <p>The number I get from calc/excel is 3.19 (<code>POISSON(6,2.16,0)*100</code>).</p>
  
  <p>Am I using the python's poisson wrong (no pun!) or am I missing something?</p>