http://www.nytimes.com/2016/07/04/world/middleeast/drone-strike-statistics-answer-few-questions-and-raise-many.html?smprod=nytcore-iphone&smid=nytcore-iphone-share
see also:
FACT SHEET: Executive Order on the US Policy on Pre & Post-Strike Measures to Address Civilian Casualties in the US Operations Involving the Use of Force & the DNI Release of Aggregate Data on Strike Outside Area of Active Hostilities whitehouse.gov