title: how to calculate a rolling average based on a column in spotfire
body: <p>I have a data set where you have a Document Property that Selects "items", each "item" has a particular "usage days". I want to calculate an output of "Moving Average" for 1 or more selected items. the data for the moving average lives under a column named "usage days".</p>
  
  <p>How do I calculate this taking into account the "selected date of my choice" and the rolling average number of days of my choice.</p>
  
  <p>Do you have particular ideas of how I can perform the calculation i.e. in a calculated column or a text field?</p>
  
  <p>Car/        Trip    /  Start Date/      End Date    /          Days on trip</p>
  
  <p>1           AB123   /   2       /     6/07/2013 </p>
  
  <p>1           AB234   /  29/07/2013   /   6/09/2013   / 42</p>
  
  <p>1           AB345   /6/09/2013  /28/09/2013 /22</p>
  
  <p>1           AB456   /29/09/2013 /21/10/2013 /23</p>
  
  <p>2          AB567    / 26/10/2013    / 12/11/2013    / 22</p>
  
  <p>2          AB678    /12/11/2013 /8/12/2013  /26</p>
  
  <p>[The rows above have an example of the problem (sorry couldn't paste an image because im new), I want to calculate the %usage of the Car and or cars for a selected range of time e.g (Select date range JUlY to AUGUST then (#of days on trip for car 1and 2)/#on days in that period)/2*100]</p>