You are correct - the WHERE statement (as the same with the IF stmt) will only interrogate the current observation. You need to use PROC SUMMARY/MEANS to generate a file containing the MIN and MAX as new variables, and then one option is to MERGE the resulting file back into your SAS file - then you have the MIN and MAX value range (again, as unique SAS variables created in the MEANS/SUMMARY step) to use in your WHERE statement.
Scott Barry
SBBWorks, Inc.