title: Adding/Combining Standard Deviations
body: <p><strong>Short Version:</strong><br>
  Can StdDevs be added/combined? i.e.</p>
  
  <pre><code>if StdDev(11,14,16,17)=X and StdDev(21,34,43,12)=Y  
  can we calculate StdDev(11,14,16,17,21,34,43,12) from X &amp; Y
  </code></pre>
  
  <p><strong>Long Version:</strong><br>
  I am designing a star schema. The schema has a fact_table (grain=transaction) which stores individual transaction response_time. The schema also has an aggregate_table (grain=day) which stores the response_time_sum per day.<br>
  In my report I need to calculate standard deviations of the response time for a given timedimension, say day, week, month etc. How can I calculate the StandardDeviation using the aggregate_table instead of touching the huge fact_table?</p>