Add string formula as parameter

Hi all,

         I have to add a parameter value which is a string. I have field called delivery status, which is based on some calculation.We have 3 values for delivery status like - fully delivered, partially delivered and not delivered. when i try to add in record selection formula , this calculated field is not shown. How can i display these 3 there values as a drop down (parameter).

Regards,

Akram Ali Khan