This would be better expressed as
```clojure
(reset! freq-by-county
  (into {}
         (map (fn [s] 
                   [(build-key (aget s "state") (aget s "county"))
                    (/ (aget s "total") (aget s "sightings"))]
          stats))
```

caveat: I coded this in the comment window so don't expect it to work ;)