How do I assign a value to a drop down field choice?

How do I assign a value to a drop down field choice?

I am using a drop down field  to record the status/outcome of a record  and need to assign a value depending on which status is chosen. 

If the status is Sold, Proposal or Cancelled, I need the value to be 1, or if the status is Assigned or Reset, I need to value to be 0. This value will be input into another field in this form, which will be summed and used in a pivot report.