I have created a subreport.  In the "Subreport Links" window, I've selected the field in my main report that I want to link to.  After checking the box, "Select data in subreport based on field", I observe that the field from the subreport I wanted to link to is omitted from the list. 

Nevermind, I've resolved the issue.  The two fields were not of the same data type.  Though the main report field consisted almost entirely of numbers, there were a couple of strings as well, which threw things off.