title: Calculated Field error involving aggregates
body: <p><strong>Problem:</strong> The sheet has a calculated field named 'Average % Delta' and the field is required at date level.
  On using the average function to figure out the average value of the calculated field, it throws a<a href="https://i.stack.imgur.com/nouNs.png" rel="nofollow noreferrer">enter image description here</a>n error. (an aggregate function cannot be aggregated further)
  Is there any way around this error?
  The calculations should hold for multiple selections in the filter too.</p>
  
  <p>Similar error repeats while trying to find the standard deviation too.</p>
  
  <p>link to tableau workbook : 
  <a href="https://drive.google.com/open?id=1jtpy9fu0UvinMz-56dI5_je8m7AZ-jnp" rel="nofollow noreferrer">https://drive.google.com/open?id=1jtpy9fu0UvinMz-56dI5_je8m7AZ-jnp</a></p>