How many shipments does each driver deliver per month on average?
shipment data was for 24 months in 2016 and 2017 respectively; deliver per month on average refers to Divide(Count(ship_id), Multiply (24, Count(driver_id)))
SELECT CAST(COUNT(*) AS REAL) / (12 * COUNT(T2.driver_id)) FROM shipment AS T1 INNER JOIN driver AS T2 ON T1.driver_id = T2.driver_id