What is the average population of the wards where apartment crimes have been reported without arrests?
apartment crime refers to location_description = 'APARTMENT';  without arrest refers to arrest = 'FALSE'; average population = AVG(Population)
SELECT AVG(T2.Population) FROM Crime AS T1 INNER JOIN Ward AS T2 ON T2.ward_no = T1.ward_no WHERE T1.location_description = 'APARTMENT' AND T1.arrest = 'FALSE'