How do I sum the number of occurences of drop down values?

[mqbui]

To clarify:

 

1) I have a database with multiple records.

2) There is a drop down box with one of the values being "x"

3) I want to query based on a period of time to find the number of "x" related records in the drop down field.

Question: How do I create a calculation to add up the number of "x" occurences and be able to pass that value for further use?

 

Any help is appreciated.

 

Thanks,

Marc