The Oregon Health Division has a spreadsheet of radon levels tested around Portland. I created the graphs below using their data.  The general recommendation is that any home testing over 4.0 pCi/l should have radon mitigated as it is the second leading cause of lung cancer behind smoking.  It is important to take both graph into consideration.  In 97268, 100% of homes test high but it was one of one.  In other zip codes there were over 100 tests so the data is more meaningful.  The link above has data for all of Oregon but I only used Portland.